CA1313228C - Frequency converter - Google Patents

Abstract

Abstract:
A frequency converter suitable for a multi-channel broadcast signal receiver is provided. The frequency converter includes a pre-amplifier with an AGC control terminal, first and second local oscillators, a first mixer for converting a first-RF frequency of the broadcast signal to a second-RF frequency using a first oscillation signal output from the first local oscillator, a second-RF signal amplifier with an AGC control terminal for amplifying the second-RF signal, a second mixer for converting the second-RF
frequency of the second-RF signal output from the second-RF
signal amplifier to a third-RF frequency using a second local oscillation signal output from the second local oscillator, and an AGC circuit for feeding back to the pre-amplifier and the second-RF signal amplifier the AGC signal which is obtained from the third-RF frequency signal as a conversion output from the second mixer for simultaneously controlling the gains of both the pre-amplifier and the second-RF signal amplifier.

Description

3~3~

Frequency converter Backaround of the Invention This is a division of application Serial Numker 513,883 Filed Jul~ 16, 1986.
Field o~ the Invention Thi~ invention relates to a frequency converter, and more particularly to a frequency converter suitable for a multi-channel broadcast signal receiver.
Description of the Prior Art Generally, the basic function of a frequency converter is to convert frequencies of received broadcast signals into prescribed frequencies. In this frequency conver~ion operation, although it is necessary to take a suffirient signal-to-noise ratio (S/N) into account to obtain the required power gain, the fre~uency conver~ion operation must be performed that a prescribed selectivity can be maintained.
Also, to avoid signal distortion, such as cross-modulation distortion, the gain of the fxequency converter must be controlled. However, it has been difficult to eliminate the signal distortion and improve the S/N simultaneously since an improvement in one results in a degradation of the other.
Therefore, the prior art faced a problem of how to suppress the signal di~tortion and prevent a ~imultaneous deterioration of the S/N of the freguency conver~er.
Apart from receivers for conventional television broadca~ing, this problem also affect~ receiver~ for CATV
broadcasting, which is a multi-channel broadcasting with many ~ransmission channels.
In a CATV converter which functions as a tuner-converter for a CATV receiver, channel frequencie~, i.eO, first-RF signal frequencies of received CATV broadcast signals, are first converted into corresponding second RF
signal frequencies which are higher than the first-RF signal fr~quencies of the raceived CATV broadcast sign~l~ by a first _ 2 - ~3~32~

mixer (up converRion) and then selected by a second-RF tuned amplifier and converted into a prescribed televi~ion channel frequency representing a vacant channel (non-broadcast channel) among a general television broadcast channel band1 e.g., the VHF band, or the UHF band by a second mixer ~down conversion). This type of frequency converter i~ known as an up-down frequency converter because it first converts the first-RF signal frequency into-the second-RF frequency which is higher than the first-RF signal fr~quency and then convert~ the second-RF frequency into the pre~cribed televi~ion channel frequency which is lower than the first-RF
signal frequency.
The frequency conver~ion as described above is carried out on the respective multiple C~TV broadcast ~ignals, while the received CATV broadcast signals or the first-RF ~ignal~
are transmitted through a coaxial cable and applied to the frequency converter. Levels or intensities of individual channel signals of the CATV broadcast channels band are not always the ~ame. Therefore, a tilt amplification characteristic can be employed at a line repeater which is pro~ided in a transmi~sion line for a reception terminal, i.e., the frequency converter, to make its gain vary for the lower and higher tran~mi~ion channel ~requencias. The tilt amplification characteristic of the line repeater is determ~ned by responding to level deviations among channels in tha roception terminal, signal di~tortion in the -transmi~ion system, etc. That is to ~ay, the tran~mis~ion characteriatics of the tilt amplification characteristic of the line repeater hould be determined by taking into account the signal distortion a~d the S~N of frequency convert~r in the reception terminal.
To enable the prior art ~o be described wlth ~he aid of diagrams, the figures of the drawings will first be listed.
Figuxe 1 is a circuit diagram ~howing an e~bodiment o~
the frequency convert~ according to ~he pre~ent lnvention;

_ 3 ~ 3~

~ igure 2 i8 a graph illu~trating the GR to AGC voltage characteristics;
Figure 3 is a graph illustrating the NF to GR
characteristics;
Fi~ure 4 is a graph illustrating the second order distortion to GR characteristic;
Figure 5 is a graph illustrating the cxoss-modulation distortion to GR characteristic; and Figure~ 6 and 7 are circuit diagrams showing conventional circuits.
Figure 6 shows a circuit of a convsntional up-down frequoncy converter. In the figure, a plurality of C~TV
broadcast channel signals are applied ~o an input terminal 1.
Then plurality of ChTV broadcast channel signals are inputted to a first mixer 4 via a band pass filter (BPF) which comprises of a high pass filter (HPF~ 2 and a low pass filter (LPF) 3. Fir~t mixer 4 al~o has applied an output of a first local oscillator 6 via an amplifier 5. The frequencie~ of the input CA~V broadcast ~isnals are raised in first mixer 4, respectively/ to frequencies each higher by a first oscillation fre~uency of first local oscillator 6. The frequency-con~erted signals, i.e., second-RF signals from first mixer 4 are input to ~econd mixer 10 Vi2 a first frequency gate which compri~es a BPF 7, a second-RF signal amplifier 8 and a BPF 9. The first freguency gate passes throu~ it a signal with a prescribed second-RF frequency of ~the.~econd-RF signals to second mixer 10~ Second mixer 10 receive~ the output of a second local oscillator 11. Second mixer 10 lower~ the prescribed second RF frequency of the signal that passed through the first frequency gat2 to a prescribed frequency which corresponds to a vacant channel (non-broadcast channel ? among the aforementioned conventional television broadcast channel band, 9.g., the VHF band or the UHF band, in using the oscillation output of second local ~ 3~ 3 2 2~

oscillator 11. The frequency-converter signal i~ output from output terminal 13 via output BPF 12.
As described above, it is generally desirabla that the frequency convert~r should not deteriorate its noise figure (NF) characteristics and should suppress any si~nal distortion occurring therein.
When a non-linear signal distortion occurs in an amplifier, generally there i8 a following relationship between an input signal voltage and an output voltage of the amplifier.
m Ye = ~ Kn-en ,., (1) n=l wherein, Ye : Output signal voltage of amplifier e : Input signal voltage of amplifier Kn : Coefficient presenting a linearity of amplification the amplifier n : Order of signal distortion Although the non~linear signal distortion occurs to fairly high orders a~ seen from Equation (1) r only the second order distortion component (n = 2) and the third order distortion component ~n = 3) need to be considered for practical u~e. When the amount of the signal distortion given by Equation (1) rises in an amplifier constituting the frequency converter, a cross-modulation disturbance and a baat disturbance occur. The degree of the cross~modulation disturbance i8 proportional to kha square of khe amplitude of the signal which interferes with the desired signal.
Moreover, the cross-modula~ion disturbance becomes grea~er as number of the received broadca~t channal signal~ increases.
On the other hand~ the beat disturb nce occur~ when signal distortion3 occurring for a plurality of the received broadcast channel signals are present in the tele~ision broadcast channel band: For redllcing the effect of the _ 5 ~ 322~

cro~-modulation distortion and the beat disturbance, pre-amplifiar 14 could be removed from the circuit arrangement shown in Figure 6. ~owever, then a carrier-to-noise ratio (C/N) at the fxequency converter wor~ens, due to the lack of the pre-amplifier.
The C/N is generally expre~sed by C/N~dB] Y ei[dB u~ - NF[dB] - 0.8[dB] .,. (2) Also, a total amount of the C/N is given as follows:
C/Nm[dB] = C/N~dB] 10 log10 m[dB~ ( 3 ) wherein m represents the number of amplifier stages connected in cascade. As seen from Equation (3)r the total amount of the C/N, i.e., the C/Nm, is inversely proportional to the number of amplifier stages in cascade; m. In other words, when m number of amplifier stage~ of the same perfoxmance are connected in ca~cade, the C/~ of the frequency converker worse~g by 10 log10 m[dB]. Therefore, when m number of amplifier stages ar0 connected in ca~c~de, each amplifier stage requires for it~ input signal a level of ei, given in the following Equation ~4), in order to maintain the C/N in value the ~amo as when only one amplifier stage ia used.

ei [dB Ul = emin[dB u] ~ 10 log10 m ... (~) wherein emi~ rspresents tha lowest signal input level which i~ obta~ned using Equation (2).
It i~ clear from Equation (4) that, in order to obtain the C/N over a prescribed value, the input signal is required to be at a sufficient level over a prescribed level.
Therefore, the inpu~ signal level for the frequency converter must be set to an optimum level to satisfy both re~uirem~nts of low signal distortion~ and high C/N~
Tn the conventional frequency converter 3hown in Figure 6, since no pre-amplifier is providQd prior to first mi~er 4, -- 6 - ~ 2~

although a lower proces~ed ~ignal level i8 de~irable fox reducing the ~ignal distortion, the C/N is deteriorated since the input signal level is insufricient to satisfy the prescribed C/N required in the rear stage amplifier, e.g., second-RF amplifier 8.
For resolving the problem, a pre-amplifier i~ provided prior to the first mixer, for example, in a position between HPF 2 and LPF 3 which are shown in the Figure 6. This pre-a~plifier i8 employed a~ ~he cost of increa~ing the signal distortion, such as the cros~-modulation distortion.
Figure 7 is a circuit diagram showing the construction of thi3 type of circuitl and it differs from the circuit in Figure 6 in that ampliier 14 is provided. Amplifier 14 is generally called a pre-amplifier. It is provided for preventing the deterioration of the C/N of ~he frequency converter.
In the frequency converter shown in Figure 7, pre-ampliier 14 amplifies the input signal to a required level, given by Equation (4), and contribute~ to the obtaining of the prescribed C/N. On the other hand, the econd and the third order distortions are increased.
That is to sayl although the CtN i5 improved, cross-modulation distortion will occur if there i5 non-linear distortion in pre-amplifier 14. Thus it is necesaary to control the gain of pre-amplifier 14 so that the ~i~nal distortion i~ not increased by exce~sive gain.
- - When an m number of amplifier stages are connected in ca~cade, if power gains of the respective amplifier stages are tak~n as Gl, G2, .. Gm and ~he NF~ of the re~peotive amplifier ~tages are taken a~ NFl, NF2, ... NFm, the total noise figure NFt is expressed by NF2 ~ L_ _L ~ + ~ B~_ _L
NFt = NF1 + Gl Gl.G2 Gl.G2... Gm~l (5) Thus for improving the ~F and ~he C/N, i~ is advantageous to heightèn the gain o the amplifiers in the ` _ 7 _ ~313?,2~

raar ~ta~es of the frequency converter. On the oth~r hand, for uppressing the ~ignal distortion, it is desirable to heighten the gain of the amplifiers in the front stages o the frequency converter. ~herefore, with respact to the gains of the am~lifiers, the NF or the C/N characteristic of its fre~u0ncy converter and the signal di~tortion characteristic have respon~es inconsistent with each other.
In the conventional frequency convarter as shown in Figure 7, sither pre-amplifier 14 or second-RF amplifier 8 is made 80 that its gain may be controlled automatically in re~ponse to the output of the frequency converter. That is, an automatic gain control (AGC) is performed in one of pre-amplifier 14 or second-RF amplifier 8.
If the AGC i5 carried ou~ in second-RF amplifier 8, a sufficient level of the input signal must be maintained over the level which satisfies E~uation (4~, so that the C~N will not be exce3sively deteriorated. However, signal distortion becomes severe when the lPvel of the input signal exceed~ a predetermined level in con~unction with the AGC. This is becau~e the AGC i~ carried out for the signal from ~irst mixer 4, in which the si~nal distortion has occ~rred due to non~linear characteri~tic elements of first mixex 4 for effecting the frequency conversion.
Moreover, in the case when the AGC is carried out in pre-amplifier 14, there is a limit to the extent of gain reduction (GR) due to the AGC, because pre-amplifiar 14 i~, for example, a 55-450 MHz broad band amplifier. Thus ~he GR
for pre-amplifier 14 cannot be expected to suffici~ntly suppress the signal distortion.
Thus, in the conventional frequency converters shown in Figures 6 and 7, since both the C/N characteristic and the distortion characteristic are prescribed, recently there has been a problem in that it is difficult to control the signal gain for those levels.

- 8 - ~3~22~

Summary_of the Invention An object of the present invent,ion is to pro~ide a frequency converter which can control the signal level at an optimum value suitable for both the C/N characterist.ic and ~he signal distortion characteristic.
These and other ob~ects are achieved in a frequency converter comprising: an input terminal for receiving a broadcast signal with a first fre~uency; a first variable gain amplifier for amplifying said broadcast signal applied from said input terminal comprising a first transistor connected in a grounded emitter coniguration and a second transistor connected in a grounded base configuration; a local oscillator for generating a local oscillation signal; a frequency mixer circuit for converting said first frequency of said broadcast signal applied from said first variable gain amplifier to a second fxequency using said local oscillation signal output from said local oscillator; a second variable gain amplifier for amplifying a signal with said second frequency applied ~rom said frequency mixer circuit; and an AGC circuit means for feeding back to said first and second variable gain amplifiers an AGC signal which is obtained from a signal output from said second variable gain amplifier.
Descxi~tion of the Preferred Embodiment~
~ mbodiments of the pre~ent invention will now be described in detail with raference to the accompanying drawings, namely, Figures 1 to 5.
- Referring now to Figure 1, there is shown the circuit diagram of a frequency converter according to the present invention. The circuit diagram h~ws an example of a circuit which carries out the AGC for an up-down ~requency converter which receives multi-channel broadcast signals~ for example, CATV broadcast signals.
In Figure 1, a plurality of broadcast signals, e.g., the CATV broadcast channel ~ignal~ are input to an input 9 - ~ 3 ~

terminal 100. Then the plurality uf CATV broadcast channel signals are applied from input terminal lO0 to a first mixer 400 via a first signal transmission circuit. The first signal transmission circuit comprises of an HPF 200, a pre-amplifier 140 and a LPF 300. First mixer 400 also receives an oscillation output of a first local oscillator 60 via an amplifier 50. The input CATV broadcast channel signals have their frequencies raised in fir~t mixer 400, respectively, to frequencies each hiyher by the first local oscillation output of first local oscillator 60. The frequency-converted signals, i.e., second RF signals, from first mixer 400 are input to a second ~ixer 110 via a second-RF signal tuner circuit 500 which comprises a BPF 70, a second~RF signal amplifier 80 and a BPF 90. The second-RF signal amplifier 500 passes through it a signal with a prescribed second-RF
frequency of the second-RF signals to second mixer 110.
Second mixer 110 is supplied with an oscillation output of a second local oscillator 111. Second mixer 110 lowers the prescribed second-R~ frequency of the signal that passed through second-RF signal amplifier 500 to a prescribed frequency which corresponds to a vacant channel (non-broadcast channel) of the aforementioned conventional television broadcast channel band, e~g.l the ~HF band or the UHF band, in using the oscillation output of second local oscillator 111. The frequency-convarted signal is output from output terminal 113 via an output BPF 112.
In amplifier 140, transi~tors Ql and Q2 are connected in cascade. Transistor Q1 is connected in a grounded-emitter configuration amplifier, while transistor Q2 i~ connected in a grounded-base configuration amplifier. An output circuit of the amplifier of transistor Ql is connected to an input circuit of the amplifier of transistor Q2 so that an input capacitance of the amplifier of transiRtor Q2 is reduced.
This makes the amplification characteristic of the cascade connected amplifier flat over a broad band. The broad band - 10 - 13132?,,P) a~plifica~ion is enhanced by feeding back an output of transistor Q2 to the input circuit of Ql via a feedback circuit composed of capacitors Cl and C2 and a re~istor R1.
The feedback operation carries out a broad band compensation.
A series circuit composed of a capacitor C5 and a resistor R7 is connected between the collector of transistor Q1 and a ground for preventing an undesired oscillation in the cascade connection amplifier. Moreover, base bias voltages for tran~istors Q1 and Q2 are applied by dividing a power supply voltage +B with a series circuit of resistors R2, R3 and R4 connected between a power supply terminal and the ground potential circuit. For the collector of transistor Q2, the bias voltage is applied via a choke coil L1 which prevents undesired high-requency signals from getting into the bias circui~. The undesired high-frequency signals are by-passed via a capacitor C3 connected between the bias circuit and the ground potential circuit.
The output of pre-amplifier 140, composed of the cascade amplifier of transistors Ql and Q2, is applied to first mixer 400 via ~PF 300 after its DC current component has been eliminated by a coupling capacitor C4 connected in serias with tho output circuit of the grounded-base coniguration amplifier of transistor Q2.
Resistors R4, R5 and R6, which are connected to the output side of pre-amplifier 140, essentially compo~e a ~
(pi)-network attenuator fox an AC signal and contribute to suppress the signal distortion that would occur in following stage first mixer 400 due to non~linear characteristic elements for a frequency conversion at first mi~er 400. The suppression of the signal distortion i9 further ensured by means of following stage LP~ 300, which prevents the undesired high-frequency signal ~rom getting into first mixer 400.

3 ~ ~ ~

In addition, pre-amplifier 140 has an AGC terminal P1 for receivlng an ~GC signal, which will be explained in detail later.
The frequency conversion at first mixer 400 is effected by an addition of the broadcast signal (first-RF sisnal) and the first local oscillation signal at a diode bridge composed of diodes D10, D20, D30 and D40. The first-RF signal~ are applied to the diode bridge through transformers T10 and T20 which are connected in series with an input circuit of first mixer 400. The first local oscillation signal is applied to the diode bridge thxough a tap of a secondary winding of transformer T20. The second-RF signals thus conver~ed by the diode bridge are output through a BPF composed of a transformer T30, an inductor ~10, and capacitors C10 and C20 in fir3t mixer 400. The second-RF signals output from first mixer 400 are applied to second-RF tuner circuit 500 composed of BPF 70, second-~F signal amplifier 80 and BP~ 90 as de~cribed above.
Second-RF tuner circuit 500 has a prescribed tuning frequency, i.e, a prescribed second-RF signal frequency, which corresponds to a desired CATV broadcast channel frequency 80 that only the prescribed ~econd-RF signal is effectively transmitted therethrough. BPF 70 is of a ~/4 type compo~ition with a steep band pass filtering characteristic. Therefore, BPF 70 selects a signal having the tuning frequency, i.e., the prescribed second RF signal frequency among the plurality of the second-RF signals from first mixer 400. The prescribed second-RF signal thus selected is amplified in second-RF signal amplifier 80. The s~lection of the prescribed second-RF ~ignal is most effected by BPF 90.
Second-RF signal amplifier 80 is composed of an FET Q3 in order to suppress the third order distortion. The signal, i.e., the prescribed second-RF signal appli~d thereto~ has been amplified in pre-amplifier 140 to the level ~uch that _ 12 ~ 3~2~

the C~N of the signal is not deteriorated as expressed in Equation (4). Moreover, second-RF signal amplifier 80 has an AGC terminal P2 for receiving the AGC signal. That is, FET
Q3 receives the AGC signal at its second gate G2 so that the gain of second-RF signal amplifier 80 is controlled. The gain reduction (GR) in second-RF signal amplifier 80 due to the AGC is made greater than ~he G~ in pre-amplifier 140 for improving the NF and the C/N in the frequency converter.
This is because a greater GR in second stage amplifier is advantageous for improving the NF and the C/N when compared to the GR in the first stage amplifier, as aforementioned and seen ~rom Equation (5). Moreover, the greater GR in second-RF signal amplifier 80 is necessary for suppressing the cross-modulation distortion which would occur at another frequency conversion in the second mixer 110.
The gain controlled output signal from second-RF signal amplifier 80 is input to BPF 90 via a filter composed of a trans~ormer T40, which is of also a ~/4 type composition with a steep band pass filtering characteristic due to inductive coupling, and then via an impedance matching circuit composed of inductors L20 and L30 and a capacitor C30. The prescribed second-RF frequency signal from second-RF signal amplifier 80 has the undesired frequency components further reduced by the filter composed of transformer ~40 and BPF gO, ~o that the ~electivity for the prescribed second-R~
frequency signal is sufficiently raised.
- In second mixer 110, the pre~cribed ~econd RF frequency signal i8 lowered in frequency to the prescribed frequency which corresponds to the prescribed channel of the conventional t~levision broadcas~ channel as mentioned above, in using the second local oscillation ou~put o second local oscillator 111. The prescribed television channel frequency signal thus converted is applied to a television recei~er through output terminal 113 o the frequency converter. In thQ television receiver, the prescribed television c~annel - 13 ~ 2 2 ~

fr~quency 3ignal from the fre~uency converter is again lowsred in frequency to an intermediate frequency (IF) in the normal manner. The IF signal is applied to an AGC signal detection circuit 116 through a surface wave filter 114 and an IF signal amplifier 115. Surface wave filter 114 rises the selectivity for the prescribed IF signal by its steep band pass filtering characteris~ic, while IF signal amplifier 115 amplifies the IF signal.
In AGC detection circuit 116, an AGC siqnal is detected from the IF signal by well-known technique, for example, a peak detection which detects signal peaks. The AGC signal thus detected is supplied simultaneously to AGC te~minals P1 and P2 of pre amplifier 140 and second-RF signal amplifier 80 so that an AGC loop is fonmed.
The AGC in the frequency converter shown in Figure 1 will be now explained in detail. The frequency conversion is carried out by means of a hekerodyne operation in irst mixer 400. For suppressing the signal distortion which occurs due to non-linear characteristic element~ for the frequency conversion, the diode bridge comprised of diodes D10, D20, D30 and D40 i~, for example, employed in first mixer 400 as described above. However, the frequency conversion is accompanied by a conversion loss, so that the signal to be supplied to second-RF signal amplifier circuit 80 becomes insufficient to prevent the C/N from being severely deteriorated.
l'herefore, pre~amplifier 140 is provided in front of first mixer 400 in order to make the C/N no worse than the C/N of the received signal on input terminal 100. However, if the gain of amplifier 140 exceeds a predetermined value~
the signal distortion as obtained by Equakion (1) will increase over a predetermined value due to the non~linearity of ~mplifier 140 itselfO
As is well-known, the non-linearity of the amplifier itself gives rise to undesirPd high-frequency signals, cross-- 14 _ ~3~3~`~J~

modulation distortion signals etc. The undesired high-frequency signals, cross-modulation distortion signals etc.
then interfere with the desired signal. The non-linear distortion creates a serious problem when there are a plurality of broadcast channels such as the conventional television broadcast channel waves or the CATV broadcast channel wave~.
Therefore, it is necessary for the gain of pre-amplifier 140 to be controlled to a degree such that the non-linear distortion is below an acceptable low level, ~nd also to have the gain such that the C/N will not be badly deter~orated in processing the signals in the following stage. For that purpose, pre-amplifier 140 is provided an AGC terminal Pl so that its gain is controlled to an optimum degree to not worsen the non-linear distortion below ~he acceptable level.
In the cascade amplifier of pre-ampliier 140, the grounded-emitter configuration amplifier composed of transistor Ql is further provided an impedance circuit, such as is well-known. The impedance circuit is comprised of a DC
impedance c.ircuit and an AC impedance circuit respectively for establishing prsscribed DC and AC emitter bia~es for the emitter of transistor Q1. The DC impedance circuit comprises resistors R51 and R52 connected in serie~ between the emitter of transistor Q1 and the ground potential circuit, while the AC impedance circuit comprises capacitors C50, C60 and C70 connected in series betweell the emitter of transistor Ql and the ground potential circuit. In addition, the AC impedanc~
circuit is provided a diode, for example, a PIN diode D50, connected in series between capacitors C60 and C70 at a forward bias condition. Then the aforementioned AGC terminal P1 i~ connected to the anode of PIN diode D50.
The gain of the grounded-emitter configura~ion amplifier composed of transistor Ql which composes the cascade amplifier is controlled in accordance with the ~ 31322~
~ 15 -emittsr impedance. On the other hand, the capacitive impedance of PIN diode D50 is variable in accordance with the forward bias applied thereon. Therefore, PIN diode D50 can vary the emitter impedance in response to the AGC signal applied to AGC terminal P1 so that the AGC for pre-ampli~ier 140 is established. When the maximum AGC signal is applied to AGC terminal P1, it is assumed here that the gain reduction in pre-amplifier cannot be achieved (GR = 0 dB).
On the other hand, the gain reduction due to the AGC is carried out by decreasing the AGC signal so that the AC
impedance on the emit~er of transistor Q1 is increased. By the action of the AGC, the gain of pre-amplifier 140 is reduced to the optimum value and the signal distortion due to amplifier 140 is suppressed.
The AGC, in combination with the capacitive AC
impedance circuit to the emitter of transistor Q1, i5 effective for suppressing the non-linear distortion which occurs in pre-amplifier 140 itself. Moreover, since the optimum gain ad~ustment is caxried out at a stage prior to first mixer 140, which is composed of diodes D10, D20, D30 and D40 which are the non-linear characteristic elements, the deterioration of the NF expressed by Equation (5) can be controlled to not worsen below an acceptable level.
Furthermore, the AGC for pre-amplifier 140 is e~fective for compensating in advance the conversion loss in the first mixer 400. Therefore, second-RF signal amplifier circuit 80 is able to amplify the prescribed second-RF signal to a le~el en~uring the C/N over a predetenmined degree but without increasing the cross-modulation distortion over the acceptable degree.
The AGC for second-RF signal amplifier 80 will now be described in detail. The AGC is perform~d by applying the ~GC signal to second gate G2 of FET Q3 in second-RF signal amplifier 80. The AGC signal alters the bias voltage of .

~ 3 ~

second gate G2 of FET Q3 so that the gain of second-RF signal amplifier 80 is controlled.
FET Q3 itself generates a small amount of third order distortion so that the cross-modulation distortion can be m~de smaller in second mixer 110. As expressed in Equation (5), from the viewpoint of prevention of deterioration of the NF, it is desirable that the rear stage amplifier perform a greater gain reduction compared to the first stage amplifier.
In this sense, the gain reduction (GR) of second-RF amplifier 80 is made greater than the gain reduction of pxe-amplifier 140.
The non-linear distortion in an amplifier as given by Equation (1) will now be investigated in detail. As described before, the third order distortions in the non-linear distortions become problems in practical usev In the third order distortion, there are included the cross-modulation distortion with a frequency the same as the ~requency of the input signal and a beat with a frequency different fxom the frequency of the input signal. In the second order distortion, it includes the beat but does not include the cross-modulation distortion with the frequency of the input signal.
If the cross-modulation distortion occurs, the desired channel signal is modulated by signals in other channels, so that the quality of the reproduced picture on television receiver~ will be seriously deteriorated. If the beat occurs, so-called beat stripes appear on the ~elevision receivers, so that the reproduced pi~ture will be also seriously deteriorated.
In the embodiment shown in Figure 1, the contxol o the gain reduction is carried out on both amplifier 140 and amplifier 80 by the same ~GC signal. Therefore, the frequency converter is protected from the deterioration of the NF and the C/N and also fxom the dis~urbances of beat and cross-modulation disto~tion.

~322~

Figure 2 shows the gain reduction control characteristics ~or the AGC signal in the embodiment of Figure 1. In Figure 2, graph A shows the GR characteristic at pre-amplifier 140, graph B shows the GR characteristic at second-RF signal amplifier 80 and graph C shows the total GR
chaxacteristic o~ the frequency conv~rter. As is clear from Figure 2, a particular state that the &R = 0(d~) corresponds to the AGC signal of about 9(V). And as the AGC signal reduces, the GR graphs ~, B and C increase. As is also clear from Figure 2, the GR graph A for pre-amplifier 140 is less than the GR graph B for second-RF signal amplifier 80. The relation of the graphs A and B serves to simultaneously prevent the deterioration of the NF and the C/N and the disturbances from the beat and the cross-modulation distortion. That is, it is desirable to carxy out gain re~uction in the rear stage second-RF signal amplifier 80 rather than to reduce the gain as much as possible in the front stage pre-amplifier 140, in order to amplify the input signal without causing the deterioration o~ the NF and the C/N. On the other hand, the gain of the pre-amplifier 140 is determined by taking into account the optimum signal level required for the advance compensation of the conversion loss in the frequency conversion at first mixer without increasing the non-linear distortion over the acceptable degree.
In second-RF sig~al amplifier 80, ~he deterioration of the ~F and the C/N are less even though the GR is made greater. Therefore, the GR of second-RF signal amplifier 80 is controlled to the optimum for minimizing the distortions due to the frequency conversion in the rear stage second mixer 110. In this case, the GR is controlled to a degree which does not fall below the level required in the rear stage circuit (e.g., amplifier 114 in the television receiver outside the frequency converter) in accordance with Equation (4) in order not-to suppress the deterioration of the C/N of the whole frequency converter. This type of optimum GR value L 3 2:?~$

control characteristic for the frequency converter as a whole is shown by graph C in Figure 2.
Figure 3 is a characteristic diagram showing relationships of the noise figure NF to the gain reduction GR
of the embodiment of Figure 1. Graphs A, B and C in Figure 3 also designate the characteristics o~ pre-amplifier 140, second-RF ampli~ier 400 and the whole circuit of the frequency converter in similar to Figure 2. For example, graph A in Figure 3 shows the relationship between the GR and the NF in pre-amplifier 140, while graph D in Fiyure 3 shows the relationship of the noise figure NF to the gain reduction GR of the conventional frequency converter.
As seen from Figure 3, although the NF of pre-amplifier 140 would be deteriorated by an excessive gain reduction, the GR may be increased to a degree at which the signal distortion in pre-amplifier 140 does not exceed an acceptable degree. Since the worsening of the NF will improve as the GR
becomes greater, it is possible to increase the GR in a state in which the worsening of the NF will lessen. As seen from graph B which shows a relationship of the noise figure NF to the gain reduction GR of second-RF signal amplifier 80, the NF in second-RF signal amplifier 80 is suppressed better than the NF in amplifier 140. This means that the greater gain reduction in second mixer 110 rather than pre-amplifier 140 makes it possibla to carry out gain reduction in second-RF
amplifier 80 at less NF compared with the pre-amplifier 140.
Gxaph C in Figure 3 shows the relationship of the noise figure NF to the gain reduction GR of the frequency converter as a whole. As seen from yraph C, the NF characteristic of the frequency converter is slightly inferior in compaxison with the NF characteristic for second-RF signal amplifier circuit 80 due to the signal feedback by the ~GC. However, a greater impro~ement of the distortion characteristic can be achieved by the AGC.

~ 19 ~3~2~

The non-linear distortion characteristic of the frequency converter will now be investigated in detail.
There is the second order distortion which causes the beat, as mentioned above, in the non-linear distortion. The second order distortion is caused mainly by the use of the diode element in the frequency conversion stage such as first mixer 400 and second mixer 110. The relationship between the second order distortion and the gain reduction GR due to AGC
is shown in Figure 4. As shown in Figure 4, the second order distortion fails to be severely deteriorated even when the GR
is increased. When the GR is ero, the second order distortion is about 70dB. And the NF is improved with the increase of the GR according to the AGC. The amount of the GR by ~GC is made to vary in the range of about 70dB to 25dB
in the embodiment of Figure 1.
The cross-modulation distortion characteristic of the embodiment of Figure 1 which occurs due to the third order distortion will now be investigated in detail. As seen from Figure 5, which shows the relationship between ~he gain reduction due to AGC and the cross-modulation distortion, the cross-modulation distortion is suppressed to about 80dB at the worst, no matter what the amount of GR.
Further as seen from Figures 3, 4 and 5, the frequency conversion operation and the signal amplification operation in the frequency converter of the present invention are carried out withouk inviting severe deteriorations for the variou~ inconsistent characteristics such as the charac$eristic of the NF, the C/N etc. and the characteristic of the signal distortion.
Moreover, since the gains of pre-amplifier 140 and second-RF signal amplifier 80 in the frequency converter system are simultaneously con~rolled at the optimum values in reference to both the noise figure N~ and signal distortion, the problems of signal distortion and of the noise fi~ure NF
due to the frequency conversion operation can be prevented.

':

-~ - 20 - ~3~

As described above, the requency converter according to the present invention is able ~o suppress signal distortions, such as the second and third order distortions, and simultaneously able to improve the carrier to noise ratio CtN. Thexefore, the present invention is able to provide a frequency converter suitable for receiving multi-channel broadcasts such as CATV broadcasts.
Moreover, the frequency converter according to the present invention is able to suppre~s the cross modulation distortion and the beat occurring in the frequency conversion, and is simultaneously able to prevent the deterioration of the desired signal wave or the characteristic of the carrier to noise ratio etc.
Also, the present invention is not limited for frequency converters used for the reception of CATV signals, which are multi-channel broadcasts, but it can also be applied to frequency converters for the recep~ion of other signals, including the reception of general television signals. Furthermore, the present inqention is not limited for up-down frequency converters but may also be applied to general frequency converters employing a front stage amplifier and a rear stage am~lifier such as down converters or up converters. Further the front stage amplifier in the frequency converter is not limited to the cascade configuration amplifier.

,

Claims

Claims:
1. A gain control circuit which receives an input signal and an AGC signal and produces an output signal comprising:
first and second transistors each with a base, an emitter and a collector;
a variable impedance connected to receive said AGC
signal, wherein the value of said variable impedance is controlled by said AGC signal;
said emitter of said first transistor being connected to said variable impedance;
said base of said first transistor being connected to receive said input signal;
said collector of said first transistor being connected to the emitter of said second transistor;
said base of said second transistor being connected to a ground potential; and said collector of said second transistor providing said output signal;
wherein the gain imparted by said gain control circuit to said input signal is controlled by the value of said variable impedance.
2. The gain control circuit of claim 1 wherein said variable impedance is a PIN diode.
3. A frequency converter comprising:
an input terminal fox receiving a broadcast signal with the first frequency;
a first variable amplifier gain for amplifying said broadcast signal applied from said input terminal;
a local oscillator for generating a local oscillation signal;
a frequency mixer circuit for converting said first frequency of said broadcast signal applied from said first variable gain amplifier to a second frequency using said local oscillation signal output from said local oscillator;

a second variable gain amplifier for amplifying a signal with said second frequency applied from said frequency mixer circuit; and an AGC circuit means for feeding back to said first and second variable gain amplifiers an AGC signal which is obtained from a signal output from said second variable gain amplifier;
wherein said first variable gain amplifier comprises:
first and second transistors each with a base, an emitter and a collector;
a variable impedance connected to receive said AGC
signal, wherein the value of said variable impedance is controlled by said AGC signal;
said emitter of said first transistor being connected to said variable impedance;
said base of said transistor being connected to receive said broadcast signal;
said collector of said first transistor being connected to the emitter of second transistor;
said base of said second transistor being connected to a ground potential; and the collector of said second transistor being connected to said local oscillator.
4. The frequency converter of claim 3 wherein said variable impedance is a PIN diode.