Tuning system for RF receiver

A system is disclosed for tuning a heterodyne receiver. The system preferably includes a voltage tunable preselector filter whose output is applied to a mixer, a voltage tunable injection filter for filtering a local oscillator signal and for applying the filtered signal to the mixer, and tuning control circuitry. The tuning control circuitry senses the power output from the injection filter and develops varying tuning voltages for tuning both filters until a peak output is developed by the injection filter, whereby the filters are tuned for heterodyning an incoming RF signal to a desired IF frequency.

FIELD OF THE INVENTION 
This invention relates generally to RF receivers and, more particularly, to 
a system for tuning such receivers by using voltage tunable filters. 
BACKGROUND OF THE INVENTION 
Conventional radio frequency preselection techniques, for frequencies in 
the 0.1 to 1 Ghz range, require the use of narrowband (1 to 10 MHz) 
mechanically tuned filters to provide filtering of received RF signals and 
to provide appropriately filtered oscillator injection signals. These 
filters are usually combined with an active or passive mixer and an 
optional amplifier to complete the radio receiver front end. 
Various attempts have been made to automate the tuning operation of the 
receiver's front end. For example, one conventional approach to automatic 
fine tuning employs an RF varactor tuned circuit, a frequency converter or 
mixer, a local oscillator, an auxiliary varactor tuned circuit, a phase 
discriminator and an amplifier arranged to provide the tuning control 
function. Wnile this approach may provide a form of automatic fine tuning, 
its phase discrimination scheme is undesireably complex, prone to 
temperature-induced instabilities, and limited in the frequency range over 
which it can provide tuning. 
With the advent of new semiconductor technology, it is now possible to 
construct a narrowband filter which may be voltage tuned over more than 
half an octave without any appreciable degradation to the filter's 
characteristic. The use of these voltage tunable filters as injection and 
RF preselection filters, combined with an improved tuning control system, 
can significantly enhance the performance of existing receiver front ends. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved tuning 
system for the front end of a radio receiver. 
It is another object of the present invention to provide a heterodyne radio 
receiver with a tuning system that is capable of automatically tuning the 
receiver over a wide frequency range. 
It is yet another objective of the present invention to provide a tuning 
system that incorporates narrowband voltage tuned filters in a manner that 
significantly enhances the performance of a radio receiver. 
The foregoing and other objectives of the present invention are attained, 
in accordance with the present invention, with a tuning system that 
includes a frequency converter or mixer and a voltage tunable filter for 
preselecting an incoming RF signal and applying that filtered signal to 
the mixer to be heterodyned by a filtered local oscillator signal. A 
voltage tunable injection filter filters the output of the local 
oscillator and applies the filtered output to the mixer to heterodyne the 
incoming radio frequency signal into an intermediate frequency signal. A 
tuning control means responds to the power output of the injection filter 
for adjusting both the injection filter and the preselector filter to 
appropriate frequencies for optimum heterodyning of the radio frequency 
signal into the intermediate frequency.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows, in a functional block diagram form, a tuning system that may 
be employed in the front end of a heterodyne radio receiver according to 
the present invention. The system consists of two major parts. The first 
part includes a voltage tunable preselector filter 12 and a voltage 
tunable injection filter 15. Both these filters are varactor tuned 
bandpass filters whose resonant or center frequencies may be adjusted by 
changing a bias voltage presented to the varactor. The preselector filter 
12 and the injection filter 15 may be voltage tuned across their 
respective tuning ranges. U.S. patent application Ser. No. 450,935, filed 
Dec. 20, 1982 and assigned to the assignee of the present invention, 
discloses a preferred construction for such voltage tunable filters. 
The second part of the system detects the injection filter power output and 
locks both the injection filter and the preselector filter at the 
appropriate frequencies for optimum heterodyning of the radio frequency 
signal by the mixer into an intermediate frequency (IF) signal. This is 
accomplished by a level detector 17 which detects the power output of the 
injection filter 15, and a bias control 18 which monitors the output of 
the level detector 17 and applies a correct DC bias to the preselector 
filter 12 and to the injection filter 15. The level of the bias applied to 
the filters is under control of the level detector 17 as discussed in more 
detail below. 
For correct system operation (when using a common bias voltage for both 
filters), the preselector filter and the injection filter are preferably 
selected to meet the following specifications. At any working bias level, 
the resonant frequencies of the preselector and injection filters differ 
by a constant frequency across their entire tuning range. That constant 
frequency difference is equivalent to the center frequency of the 
intermediate frequency amplifier 16. The tuning range of the injection 
filter is equivalent in absolute frequency to the desired local oscillator 
range and the preselect filter tuning range is the same, but adjusted 
either plus or minus to the frequency difference which corresponds to the 
intermediate frequency. Preferably, the passband of the injection filter 
is equal to, or narrower than the preselector filter passband, and the 
response contains substantially no passband ripple. 
The bias control described broadly hereinabove may be implemented in a 
number of ways. One such approach is illustrated in FIG. 2. Referring to 
FIG. 2, the level detector 17 is of conventional design (a crystal 
detector, for example) which detects the RF power output of the injection 
filter 15 and converts the detected RF power level to a proportionate DC 
voltage. The bias control 18 includes a conventional peak detector 20 
which detects and holds the peak of the power output of the injection 
filter as detected by the level detector 17. The output of the peak 
detector 20 is coupled to the positive input of a voltage comparator 21, 
the negative input of which is coupled to the output of the level detector 
17. As described more fully below, the comparator 21 and the peak detector 
20 cooperate to determine when the output from the level detector 17 is at 
a maximum. 
Also included in the bias control 18 is a peak decay detector 22 which 
continually monitors the output of the level detector 17 and compares this 
value with the value held by the peak detector. This latter function may 
be accomplished by using a conventional comparator. 
The bias control is further provided with a logic circuit which may be in 
the form of a flip-flop circuit 23 that is set by the comparator 21 and 
reset by the output of the peak decay detector 22. The bias source 24, 
which is under control of the output of the flip-flop, operates either in 
an active or passive state. 
In the active state, when the flip-flop is reset, the bias source operates 
as an oscillator of a type which, in this example, provides a free-running 
triangular wave output to the injection and preselection filters. In the 
passive state, when the flip-flop is set, the bias source halts its 
oscillatory action and holds its output constant at the last attained 
level. At that level, the filters should be optimally tuned and the level 
detector 17 should sense a maximum power output from the injection filter 
15. 
To heterodyne a particular radio frequency signal to an intermediate 
frequency, a local oscillator signal of desired level and frequency is 
presented to the input of the injection filter 15. The bias control 18 is 
in an active state and allows the bias source 24 to tune the injection 
filter 15 across a predetermined band which contains the local oscillator 
signal frequency. 
During the tuning process, the level detector 17 detects the changing power 
level at the output of the injection filter. As that power level 
increases, the output of the level detector 17 undergoes a corresponding 
increase which is sensed by the peak detector 20 and the comparator 21. As 
the point of peak power output from the filter 15 is passed, the input to 
the peak detector 20 decreases. Because the peak detector cannot follow 
that decrease, its output remains at a value which corresponds to the 
maximum output from the level detector 17. Consequently, the positive 
input of the comparator 21 now receives a larger voltage than its negative 
input, thus causing its output to go high and set the flip-flop 23. This 
causes the bias source 24 to go to its passive state and to hold its 
output voltage at the last attained level. A lock condition is thus 
established in which the injection filter 15 and the preselect filter 12 
are at the appropriate resonant frequencies for the radio frequency signal 
to be heterodyned by the mixer to develop the proper intermediate 
frequency. 
After the system has locked, the peak decay detector 22 continually 
monitors the output power of the injection filter. If the output power 
degrades due to the changes in uncontrollable parameters such as 
temperature instabilities, circuit leakages, filter drifts, or in 
controllable parameters such as a change in the local oscillator level or 
frequency, then the peak decay detector 22 will sense this power 
degradation, with reference to the peak power level held by the peak 
detector 20, and reset the bias source 24. The system will again lock, 
once the optimum injection filter level has been detected. 
The peak detector 20, the peak decay detector 22 and the bias source 24 may 
each be of conventional contruction. FIG. 3, to which reference is now 
made, illustrates exemplary designs for these items. 
As shown, the peak detector 20 may include a buffer amplifier 26 receiving 
the output of the level detector 17. The output of the buffer 26 is peak 
detected by a diode 28 and a capacitor 30 in the usual manner. 
The peak decay detector 22 includes a voltage comparator 32 whose negative 
input receives the output of the level detector 17. The positive input to 
the comparator 32 is received from the junction between resistors 34 and 
36. These resistors are serially coupled between ground and the output of 
the peak detector. Their function is to establish a reset level 
(determined by the ratio of resistor 34 to resistor 36) indicative of a 
point at which it is desirable for the flip-flop 23 to be reset so that 
the filters can be retuned to account for circuit leakages, temperature 
changes and the like. 
The bias source 24 may include a conventional oscillator 38 which runs 
continuously. Its output is coupled to a buffer 40 which drives a sample 
and hold circuit comprising a switch 42 and a capacitor 44. Closure of the 
switch 42 is controlled by the Q output of the flip-flop 23. 
In operation, resetting the flip-flop 23 causes its Q output to be false so 
as to close the switch 42. This allows to signal from the oscillator 38 to 
pass through the switch 42 for application to the filters 10 and 15. 
Conversely, setting the flip-flop 23 causes the switch 42 to open the path 
between oscillator 38 and the filters. In that situation, the capacitor 44 
holds the last value of the oscillator signal for application of that 
value to the filters 12 and 15. 
The foregoing discussion has described one method by which the filters may 
be tuned according to the invention. Yet another method by which the 
filters may be tuned according to the present invention is to make use of 
a bias control circuit which continually monitors the level detector 17 
output and, by means of continual feedback adjustment of the bias voltage, 
attempts to maximize at all times the power level at the output of the 
injection filter. This "dynamic" approach would be continually active and 
would yield no passive or locked state. 
With the use of the tuning system in accordance with the present invention 
as described hereinabove, the following advantages are provided. 
First, degradation from optimum receiver performance due to filter drifts 
caused by temperature instabilities is eliminated. This is made possible 
because the preselect filter and the injection filter are controlled by 
the bias control 18 such that the filter drifts due to temperature 
instabilities are common to both filters. Thus, for a particular receive 
frequency, the bias voltage applied to the filters in a locked condition 
would not only be a function of injection frequency, but the bias voltage 
will also be a function of temperature to provide automatic self 
adjustment. 
Yet another advantage of the present tuning system is that it provides 
stand-alone automatic tuning operation with no external control over a 
wide range of RF frequencies. This feature provides a standard high 
specification front end that is compatible with most IF receivers and 
local oscillator sources. 
Still another advantage of the present system is that it utilizes an 
injection filter in a dual function. First, as a frequency sensitive 
device, the filter's output is used by the control system to determine the 
correct receiver tuning. Second, as a conventional narrowband injection 
filter, it eliminates local oscillator spectral impurities such as 
harmonic distortions, and more significantly, eliminates mixer wideband 
noise desensitization and mixer matching problems which are associated 
with conventional broadband designs. 
Still another advantage of the present system is that, due to the high 
speed with which varactor tuned filters may be aligned and the simplicity 
of this system, the front end tuning time can be several orders of 
magnitude faster than the lock time for standard RF frequency 
synthesizers. The combination of RF frequency synthesis and receiver 
front-end self tuning go hand in hand to make it possible to provide a 
mid-to-high specification receiver which can perform complicated tasks 
such as frequency hopping or wide-space frequency scanning, and may be 
purchased "off the shelf" and programmed to operate on any customer 
frequency without factory or field tuning of the receiver's front end. 
Although the invention has been described in terms of an exemplary 
embodiment, it will be obvious to those skilled in the art that many 
alterations and modifications may be made without departing from the 
invention. Accordingly, it is intended that all such alterations and 
modifications be included within the spirit and scope of the invention as 
defined by the appended claims.