CA1075326A - Low-noise microwave signal generator - Google Patents
Low-noise microwave signal generatorInfo
- Publication number
- CA1075326A CA1075326A CA256,115A CA256115A CA1075326A CA 1075326 A CA1075326 A CA 1075326A CA 256115 A CA256115 A CA 256115A CA 1075326 A CA1075326 A CA 1075326A
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- CA
- Canada
- Prior art keywords
- multiplier
- frequency
- amplifier
- signal
- output
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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- Amplifiers (AREA)
- Inductance-Capacitance Distribution Constants And Capacitance-Resistance Oscillators (AREA)
Abstract
A LOW-NOISE MICROWAVE SIGNAL GENERATOR
Josef L. Fikart ABSTRACT
A microwave signal generator having improved noise spectral characteristics employs a plurality of amplifier-frequency multiplier circuits in combination with a very stable VHF oscillator. Instead of lumping the amplification and frequency multiplication together, each in a single stage, a plurality of amplifiers and multipliers are interleaved, as described herein, resulting in reduced FM noise gener-ation.
Josef L. Fikart ABSTRACT
A microwave signal generator having improved noise spectral characteristics employs a plurality of amplifier-frequency multiplier circuits in combination with a very stable VHF oscillator. Instead of lumping the amplification and frequency multiplication together, each in a single stage, a plurality of amplifiers and multipliers are interleaved, as described herein, resulting in reduced FM noise gener-ation.
Description
I~-36 ~7~3Z~
BACKGP~ND CE' THE IN~TICN
1. FI~LD OF THE INV~TICN
In micr~wave FM (frequency mDdulation) comm~nication systems capa-ble of handling frequency division multiplex telephone channels or video signals, the noise generated by local oscillators is always an important system consideration. Io satisfy the overall noise performance requirements, ` in a multihop microwave system the lccal oscillators must meet relatively stringent noise limits. For example, the FM noise of a single local oscillator in the baseband frequency range 70 kHz - 4MHz typically should be more than 83 dB below the standard 200 kHz rms FM deviation. Such noise requirements as this dictate the use of particular types of oscillator designs and pre-clude the use of others.
BACKGP~ND CE' THE IN~TICN
1. FI~LD OF THE INV~TICN
In micr~wave FM (frequency mDdulation) comm~nication systems capa-ble of handling frequency division multiplex telephone channels or video signals, the noise generated by local oscillators is always an important system consideration. Io satisfy the overall noise performance requirements, ` in a multihop microwave system the lccal oscillators must meet relatively stringent noise limits. For example, the FM noise of a single local oscillator in the baseband frequency range 70 kHz - 4MHz typically should be more than 83 dB below the standard 200 kHz rms FM deviation. Such noise requirements as this dictate the use of particular types of oscillator designs and pre-clude the use of others.
- 2. DESCRIPTION OF THE PRIOR ~RT
A number of different micro~ave signal generators suitable for use as local oscillators have been designed ~or good ahort-term stability and low FM noise generation. A survey of such oscillators and the problems enoountered with these designs is found in an article entitled, "Short Term Stable Microwave Sources", by D.B. Leeson, in The Microwave Journal, June 1970.
One of the most ccmmon type of microwave local oscillator is known as the crystal-multiplier microwave sou~ce. A quartz crystal, place1 in a VHF oscillators, is usually used as the basic frequency reference element.
Frequency multipliers, usually using varactors or step-rec~very diodes, increase the frequency of the VHF crystal oscillator to the desired micro-wave frequency. Amplifiers are also used in the oscillator-multiplier chain to over~ome the high power losses of the multipliers. A bandpass filter selects the desired frequency component and suppresses the unwanted harmonics. Incidental to this design is a microwave isolator which matches the multiplier to the filter, and thereby redu oe s reflections back into - 30 the multiplier stage.
I~C-36 t ~
Although this design is see~ m gly simple, the actual circuit can ~e quite complex . FurthermDre, FM noise is often a problem in this design because of the high level of amplification needed to drive the multiplier circuits. Iypically, a class C amplifier design is used which produces AM and PM signal distortion and shot noise. And when this noise is applied to the frequency mLltiplier, the noise frequency deviation is amplified by the frequency multiplier factor. Consequently, a noise stripping filter is commonly used at the amplifier output to reduce the FM noise. Such -~ filters add to the oscillator cost, and they have their own design deficiency, such as frequency stability. Furtherm~re, power dissipation in this design is high whether the amplifier is a class A or a class C type design. High power dissipation natura~lly leads to heat dissipation problems.
OBJECTS AND SUMM~RY OF 1~ INVENTION
It is an object of this invention to provide an improved microwa~e frequency signal generator for use in an FM microwave system.
It is further object of this invention to provide an improved microwave frequency generator which is inexpensive, has low DC power dis-sipation, and has good FM noise performance.
In accordance with the above objects, there is provided a novel design employing distributed amplifier/m~ltiplier stages. A frequency ;~ stable VHF oscillator, usually crystal controlled, provides a lcw-noise reference signal having a frequency fx~ At least two amplifier/multiplier stages generate a composite signal having at least N frequency components harmonically related to fx~ A microwave bandpass filter centered about the frequency NfX and adapted to receive the composite signal, selects the de-sired low-noise microwave signal having a frequency Nfx~
,:~. : . .
; BRIEF DE æ RIPTION OF THE DR~WINGS
Figure 1 is a block diagram of a microwave signal generator used in the prior art which has been heretofor described.
Figure 2 is a block diagram of an improved microwave signal generator employing the features of this invention.
DEq~lLED DESCRIPTION OF THE PR~KK~u EMBODIMENT
.
Referring to figure 2, oscillator 20 is shown as a crystal .,. ~
f~
.
LC-36 ~ 32~
oscillator which generates the stable low-noise VHF signal on path 21.
Crystal oscillators are co~monly used for this purpose since they are efficient in terms of cost, size, power co~su~,ption noise per~orm3nce, and long-term frequency stability. The VHF signal (shown as 100 MHz) is amplified by a small-signal amplifier 22. This buffer amplifier 22 serves to isolate crystal oscillator 20 frcm the premultiplier 23, i.e., it provides both the oscillator and the multiplier with a good i~pedance match to prevent reflect-ions and the resulting instability. Buffer amplifier 22 also provides signal gain which is needed to compensate for the premultiplier conversion loss.
The gain of buffer amplifier 22 is normally set relatively high bu~ without causing any problems of instability between crystal oscillator 20 and pre-m~ltiplier 23. Too high a gain could result in just such an instability.
Instead of amplifying the signal level to a high power point, ~ as is done in the prior art, the VHF signal is applied to a frequency pre-- multiplier 23. The decision of how to split the total frequency multiplic-ation into two or more independent stages depends upon several factors, and each application must be handled on a case by case basis. However, the frequency premNltiplier should be at least a x3 mNltiplier. The maximum amount of multiplication dep~nds u~on the particular frequencies in~olved.
In general, boo much increase in frequency will necessitate a ~ore complex medium power amplifier design. The frequency should be kept low enough so that signal gain does not become serious design problem. e.g., a power amplifier at 300 MEz is more desirable ~han a 1 GHz amplifier. Gn the low end, too low a multiplication factor will necessitate the medium pcwer amplifier24 to generate too high a level before final ~ultiplication in the multiplier 26. m e optimum design depends upon particularly the frequencies invvlved, the circuit losses, and the total multiplication factor required.
Prem~ltiplier 23, as is true with most m~ltiplier circuit designs, utilizes a small amount of filtering on both the input and output connections~
The filtering on the output is particularly desirable since high level un-desirable frequency components could possibly overload the medium pcwer amplifier. The filtering in the premultiplier prevents this by suppressing ,~
3Z~
; the un~anted frequenc~ haxmonics. This output signal of ~ ~re~uency nl fx is amplified in the medium po~er amplifier 24 and applied to the final multiplier circuit 26 where the ; desired microwave signal is finally generated. As in Fig. l ; t~e microwave frequency signal is applied to an isolator 27 and a narrow ~andpass filter 28. There are particular multiplier design~ which would not requre such an isolator circuit, however, most multiplier designs have a broad frequency spectral output and do require some form of isolation ~itha bandpass filter. A broad~and output would have unde-- sirable harmonics which, depending upon the termination of the filter, could he reflected ~ack into the multiplier re~ulking in an insta~ility. Clearly the situation is a return loss problem rather than an inherent requirement of the system. The ~andpass filter 28 is a microwave type filter which selects the desired harmonic from the multiplier output.
` The "lumped approach" as explained in relation to Fig. 1, above, has inherently high FM noise due principally .~ .
~ 20 to the high noise generated at the output of the high power : amplifier 13. This noise is then multiplied in power by the multiplier factor of frequency multiplier lS. It can be quantitatively sho~n that the distributed approach of Fig. 2, has superior noise performance in theor~ and in practice, all okher factors being the same. To understand why this is true the two designs should be compared.
,, ~ .
For a fair comp~rison of two designs, it is assumed that the corresponding gains, losses, and levels of the lumped and distributed designs are the same. First of all, it can be assumed that buffer amplifiers 22 and 12 contribute little :to`the overall FM noise. Also, premultiplier 23 ~shown as a tripler in Fig. 2) introduces relativel~ little _ 5 _ ' ~ ~ r -.; .
FM noise to the''desired signal.
The amount of frequency multipIication required in bo~h designs is the'same,' consequentl~ the signal'loss incurred in the'multiplier stages is necessarily the same in ~oth designs. In the distributed approach of Fig. 2, this loss is distributed between the two multiplier ~23 and 26) roughly in e~ual proportion to the amount of multiplica-tion exhibited by each. It follows then ~hat the input level of ampli~ier 24 is roughly 30~ below the input level of power c~plifier 13. And since the gains of both amplifiers must be approximately the same, the output level of amplifier 24 is the same 30~ below the output level of power amplifier 13. Amplifier 13 is typically a class-C design owing to the high power levels necessary to overcome the high losses in multiplier 15. However, since amplifier 24 requires 30~
less output power, a muc~ more linear circuit design can be used for amplifier 25. Therefore, the noise contributed by amplifier 24 is less than the noise introduced by power amplifier 13. And, more importantly, the FM noise which is generated is then multiplied ~y a smaller factor, ~3 times less using the numbers of the example of Fig 2) which further lowers the FM noise over what it is in the lumped approach of Fig. l. Conseqùently, the noise generated by the distributed approach is significantly better than the noise gençrated by the lumped approach circuit.
' , " ` .
.~ .
,~
rr
A number of different micro~ave signal generators suitable for use as local oscillators have been designed ~or good ahort-term stability and low FM noise generation. A survey of such oscillators and the problems enoountered with these designs is found in an article entitled, "Short Term Stable Microwave Sources", by D.B. Leeson, in The Microwave Journal, June 1970.
One of the most ccmmon type of microwave local oscillator is known as the crystal-multiplier microwave sou~ce. A quartz crystal, place1 in a VHF oscillators, is usually used as the basic frequency reference element.
Frequency multipliers, usually using varactors or step-rec~very diodes, increase the frequency of the VHF crystal oscillator to the desired micro-wave frequency. Amplifiers are also used in the oscillator-multiplier chain to over~ome the high power losses of the multipliers. A bandpass filter selects the desired frequency component and suppresses the unwanted harmonics. Incidental to this design is a microwave isolator which matches the multiplier to the filter, and thereby redu oe s reflections back into - 30 the multiplier stage.
I~C-36 t ~
Although this design is see~ m gly simple, the actual circuit can ~e quite complex . FurthermDre, FM noise is often a problem in this design because of the high level of amplification needed to drive the multiplier circuits. Iypically, a class C amplifier design is used which produces AM and PM signal distortion and shot noise. And when this noise is applied to the frequency mLltiplier, the noise frequency deviation is amplified by the frequency multiplier factor. Consequently, a noise stripping filter is commonly used at the amplifier output to reduce the FM noise. Such -~ filters add to the oscillator cost, and they have their own design deficiency, such as frequency stability. Furtherm~re, power dissipation in this design is high whether the amplifier is a class A or a class C type design. High power dissipation natura~lly leads to heat dissipation problems.
OBJECTS AND SUMM~RY OF 1~ INVENTION
It is an object of this invention to provide an improved microwa~e frequency signal generator for use in an FM microwave system.
It is further object of this invention to provide an improved microwave frequency generator which is inexpensive, has low DC power dis-sipation, and has good FM noise performance.
In accordance with the above objects, there is provided a novel design employing distributed amplifier/m~ltiplier stages. A frequency ;~ stable VHF oscillator, usually crystal controlled, provides a lcw-noise reference signal having a frequency fx~ At least two amplifier/multiplier stages generate a composite signal having at least N frequency components harmonically related to fx~ A microwave bandpass filter centered about the frequency NfX and adapted to receive the composite signal, selects the de-sired low-noise microwave signal having a frequency Nfx~
,:~. : . .
; BRIEF DE æ RIPTION OF THE DR~WINGS
Figure 1 is a block diagram of a microwave signal generator used in the prior art which has been heretofor described.
Figure 2 is a block diagram of an improved microwave signal generator employing the features of this invention.
DEq~lLED DESCRIPTION OF THE PR~KK~u EMBODIMENT
.
Referring to figure 2, oscillator 20 is shown as a crystal .,. ~
f~
.
LC-36 ~ 32~
oscillator which generates the stable low-noise VHF signal on path 21.
Crystal oscillators are co~monly used for this purpose since they are efficient in terms of cost, size, power co~su~,ption noise per~orm3nce, and long-term frequency stability. The VHF signal (shown as 100 MHz) is amplified by a small-signal amplifier 22. This buffer amplifier 22 serves to isolate crystal oscillator 20 frcm the premultiplier 23, i.e., it provides both the oscillator and the multiplier with a good i~pedance match to prevent reflect-ions and the resulting instability. Buffer amplifier 22 also provides signal gain which is needed to compensate for the premultiplier conversion loss.
The gain of buffer amplifier 22 is normally set relatively high bu~ without causing any problems of instability between crystal oscillator 20 and pre-m~ltiplier 23. Too high a gain could result in just such an instability.
Instead of amplifying the signal level to a high power point, ~ as is done in the prior art, the VHF signal is applied to a frequency pre-- multiplier 23. The decision of how to split the total frequency multiplic-ation into two or more independent stages depends upon several factors, and each application must be handled on a case by case basis. However, the frequency premNltiplier should be at least a x3 mNltiplier. The maximum amount of multiplication dep~nds u~on the particular frequencies in~olved.
In general, boo much increase in frequency will necessitate a ~ore complex medium power amplifier design. The frequency should be kept low enough so that signal gain does not become serious design problem. e.g., a power amplifier at 300 MEz is more desirable ~han a 1 GHz amplifier. Gn the low end, too low a multiplication factor will necessitate the medium pcwer amplifier24 to generate too high a level before final ~ultiplication in the multiplier 26. m e optimum design depends upon particularly the frequencies invvlved, the circuit losses, and the total multiplication factor required.
Prem~ltiplier 23, as is true with most m~ltiplier circuit designs, utilizes a small amount of filtering on both the input and output connections~
The filtering on the output is particularly desirable since high level un-desirable frequency components could possibly overload the medium pcwer amplifier. The filtering in the premultiplier prevents this by suppressing ,~
3Z~
; the un~anted frequenc~ haxmonics. This output signal of ~ ~re~uency nl fx is amplified in the medium po~er amplifier 24 and applied to the final multiplier circuit 26 where the ; desired microwave signal is finally generated. As in Fig. l ; t~e microwave frequency signal is applied to an isolator 27 and a narrow ~andpass filter 28. There are particular multiplier design~ which would not requre such an isolator circuit, however, most multiplier designs have a broad frequency spectral output and do require some form of isolation ~itha bandpass filter. A broad~and output would have unde-- sirable harmonics which, depending upon the termination of the filter, could he reflected ~ack into the multiplier re~ulking in an insta~ility. Clearly the situation is a return loss problem rather than an inherent requirement of the system. The ~andpass filter 28 is a microwave type filter which selects the desired harmonic from the multiplier output.
` The "lumped approach" as explained in relation to Fig. 1, above, has inherently high FM noise due principally .~ .
~ 20 to the high noise generated at the output of the high power : amplifier 13. This noise is then multiplied in power by the multiplier factor of frequency multiplier lS. It can be quantitatively sho~n that the distributed approach of Fig. 2, has superior noise performance in theor~ and in practice, all okher factors being the same. To understand why this is true the two designs should be compared.
,, ~ .
For a fair comp~rison of two designs, it is assumed that the corresponding gains, losses, and levels of the lumped and distributed designs are the same. First of all, it can be assumed that buffer amplifiers 22 and 12 contribute little :to`the overall FM noise. Also, premultiplier 23 ~shown as a tripler in Fig. 2) introduces relativel~ little _ 5 _ ' ~ ~ r -.; .
FM noise to the''desired signal.
The amount of frequency multipIication required in bo~h designs is the'same,' consequentl~ the signal'loss incurred in the'multiplier stages is necessarily the same in ~oth designs. In the distributed approach of Fig. 2, this loss is distributed between the two multiplier ~23 and 26) roughly in e~ual proportion to the amount of multiplica-tion exhibited by each. It follows then ~hat the input level of ampli~ier 24 is roughly 30~ below the input level of power c~plifier 13. And since the gains of both amplifiers must be approximately the same, the output level of amplifier 24 is the same 30~ below the output level of power amplifier 13. Amplifier 13 is typically a class-C design owing to the high power levels necessary to overcome the high losses in multiplier 15. However, since amplifier 24 requires 30~
less output power, a muc~ more linear circuit design can be used for amplifier 25. Therefore, the noise contributed by amplifier 24 is less than the noise introduced by power amplifier 13. And, more importantly, the FM noise which is generated is then multiplied ~y a smaller factor, ~3 times less using the numbers of the example of Fig 2) which further lowers the FM noise over what it is in the lumped approach of Fig. l. Conseqùently, the noise generated by the distributed approach is significantly better than the noise gençrated by the lumped approach circuit.
' , " ` .
.~ .
,~
rr
Claims (6)
1. Apparatus for producing a low-noise frequency-stable microwave signal comprising:
a low-noise frequency-stable VHF signal generator providing a first signal having a frequency fx;
first amplifier means having a first amplifier input and a first amplifier output, said first amplifier input connected to said signal generator, and providing signal gain in the VHF range to said first signal;
first frequency multiplier means having a first multiplier input and a first multiplier output, said first multiplier input connected to said first amplifier output, said multiplier means generating a second signal harmonically related to fx;
second amplifier means having a second amplifier input and a second amplifier output, said second amplifier input connected to said first multiplier output, and providing signal gain to said second signal;
second frequency multiplier means having a second multiplier input and a second multiplier output, said second multiplier input connected to said second amplifier output, said second multiplier means generating a signal of frequency Nfx1 where N is a predetermined integer and Mfx is in the microwave frequency range; and a microwave bandpass filter connected to said second multiplier output, having a passband centered about Nfx, and producing a microwave frequency signal having a total noise content less than what would be produced by an otherwise equivalent amplifier-frequency-multiplier combination interconnected between said VHF signal generator and said microwave bandpass filter.
a low-noise frequency-stable VHF signal generator providing a first signal having a frequency fx;
first amplifier means having a first amplifier input and a first amplifier output, said first amplifier input connected to said signal generator, and providing signal gain in the VHF range to said first signal;
first frequency multiplier means having a first multiplier input and a first multiplier output, said first multiplier input connected to said first amplifier output, said multiplier means generating a second signal harmonically related to fx;
second amplifier means having a second amplifier input and a second amplifier output, said second amplifier input connected to said first multiplier output, and providing signal gain to said second signal;
second frequency multiplier means having a second multiplier input and a second multiplier output, said second multiplier input connected to said second amplifier output, said second multiplier means generating a signal of frequency Nfx1 where N is a predetermined integer and Mfx is in the microwave frequency range; and a microwave bandpass filter connected to said second multiplier output, having a passband centered about Nfx, and producing a microwave frequency signal having a total noise content less than what would be produced by an otherwise equivalent amplifier-frequency-multiplier combination interconnected between said VHF signal generator and said microwave bandpass filter.
2. Apparatus as in claim 1 further comprising a microwave isolator having an input and output, said isolator input connected to said second multiplier output and said isolator output connected to said microwave bandpass filter.
3. Apparatus as in claim 2 wherein said stable signal generator further comprises a quartz crystal controlled oscillator.
4. Apparatus as in claim 3 wherein said first frequency multiplier means further comprises a frequency premultiplier having at least a times three multiplication factor.
5. Apparatus as in claim 4 wherein said second frequency multiplier means further comprises a frequency multiplier having at least a times three multiplication factor.
6. Apparatus as in claim 5 wherein said first amplifier means further comprises an amplifier having a signal gain no greater than 10 dB.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA256,115A CA1075326A (en) | 1976-06-30 | 1976-06-30 | Low-noise microwave signal generator |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA256,115A CA1075326A (en) | 1976-06-30 | 1976-06-30 | Low-noise microwave signal generator |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1075326A true CA1075326A (en) | 1980-04-08 |
Family
ID=4106333
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA256,115A Expired CA1075326A (en) | 1976-06-30 | 1976-06-30 | Low-noise microwave signal generator |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1075326A (en) |
-
1976
- 1976-06-30 CA CA256,115A patent/CA1075326A/en not_active Expired
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |