Variable frequency sources

An R.F. signal generator is arranged to provide an output signal having a controlled level, by means of a feedback loop into which an additional frequency dependent control signal is injected. This additional control signal is derived from a resistor network in which particular resistors, or combinations thereof, are selected in dependence on the output frequency of the signal generator.

This invention relates to variable frequency sources and is particularly 
concerned with the amplitude of a signal provided by such a source. 
Variable frequency sources, such as R.F. frequency generators, include 
output attenuators and connecting cables associated therewith having 
impedances which include reactive components. The effect of this is that 
the power loss in the output attenuators and the connecting cables is not 
constant but rises with increasing frequency. It is known to include a 
variable gain stage which is controlled by an R.F. detector having a 
predetermined reactive impedance so that greater gain (or less 
attenuation, as the case may be) is provided in an attempt to compensate 
for the variation of output power level with frequency. Such expedients 
are, often, not sufficiently accurate and the use of R.F. detectors whose 
impedance varies with frequency introduces additional difficulties. The 
present invention seeks to provide an improved variable frequency source 
in which the output level is substantially constant as the frequency 
varies. 
According to the invention, a variable frequency source includes a variable 
gain stage for receiving a variable frequency R.F. signal and for 
controlling the amplitude of the R.F. signal in dependence on its 
frequency by means of a control signal derived from a resistor network, 
the value of the control signal being at least partly dependent of the 
selection of particular resistors in the network which are selected as a 
function of the frequency of the variable R.F. signal. 
Preferably, the variable gain stage forms part of a feedback loop which 
includes an R.F. frequency detector, said control signal also being partly 
dependent on the output provided by the R.F. frequency detector. 
Preferably again, the feedback loop includes, in addition to the variable 
gain stage for receiving a variable frequency R.F. signal, an R.F. 
detector and a dual input comparator, one input of which is obtained from 
the R.F. detector and the other input of which is derived from the said 
resistor network, the output of the comparator being used to control said 
variable gain stage. 
The output of the variable signal source is, preferably, taken off from a 
point in the feedback loop immediately preceding the R.F. detector and is 
fed via a length of coaxial cable to an attenuator. 
Preferably, there is provided an adjustable gain amplifier between said 
resistor network and said other input of the comparator so as to enable 
fine level adjustment to be achieved. 
Preferably, the resistors in the resistor network are selected in 
dependence on the most significant digits determining the frequency output 
of the variable frequency source. In all variable frequency sources, some 
form of frequency selection must be provided and where the selection is 
accomplished by means of digital keys, preferably the selection of the 
resistors is coupled to the operation of keys identifying the most 
significant digits of the output frequency value.

A variable frequency R.F. signal which may be modulated as required is 
applied to a variable gain stage 1 via a terminal 2. The variable gain 
stage 1 may consist of a variable attenuator or, alternatively, a variable 
gain amplifier. The output of the variable gain stage 1 is passed via a 
fixed gain amplifier 3 to an R.F. detector 4 consisting of a high 
frequency Schottky barrier diode. A detected signal is applied to one 
input of a dual input comparator 5 which, in this case, consists of a high 
gain differential amplifier. The output of the comparator 5 constitutes a 
control signal which is applied to the variable gain stage 1 to complete a 
feedback loop and to control the gain of the variable gain stage as 
required. The output of the variable frequency source is obtained at a 
terminal 6 via coarse attenuator 7, a length of coaxial cable 8 and a 
source resistor 9 from a point 10 in the feedback loop immediately 
preceding the R.F. detector 4. The source resistor 9 has a value of 
50.OMEGA. and represents the output impedance of the variable frequency 
source. The components 7 and 8, in particular, contain reactive impedances 
and their power losses rise with increasing frequency so that, if no 
compensation were provided, the output power level obtainable at terminal 
6 would be lower at the upper end of the output frequency band than it 
woud be at the lower end. Even so, the feedback loop, as so far described, 
serves to elliminate amplitude variations occurring in the input signal 
applied to terminal 2. 
To reduce to acceptable levels the amplitude variations in the output 
signal, two further amplfiers 11 and 12 and a resistor network 13 are 
coupled to the second input of the comparator 5. In this way, the 
effective reference level which is applied to the comparator 5 to 
determine the operating level of the feedback loop, can be altered as 
required and, in practice, it is arranged that it is altered in step with 
changes in the value of the output R.F. carrier frequency. 
Typically, an R.F. frequency s ource may provide an output carrier 
frequency extending up to 500 MHz and it has been found sufficient to 
sub-divide this range into twenty-five divisions and to provide a 
different correction factor for each division. It will be appreciated that 
each division of 20 MHz can be identified from the two most significant 
digits of the frequency value and, as the output value of the frequency 
source is manually or automatically selected, so this information is also 
used to select appropriate switches 14. One or more switches can be 
selected as required and the gain of the amplifier 12 is then determined 
by the ratio of resistor 15 to the effective resistance of the network 13. 
Fine adjustment can be provided when setting up the circuit by means of 
adjustable potentiometers 16 and 17. Two potentiometers 16 and 17 are used 
so that one potentiometer 16 can be used to set the approximate signal 
level fed to the comparator 5, and the other potentiometer 17 is used to 
adjust the overall degree of frequency compensation. The additional 
variable gain amplifier 11 can be used to introduce deliberate variations 
in the output level as required and it effectively functions as a fine 
attenuator control.