Single balanced mixer with output filter

A balanced mixer and output filter for television receivers and the like is presented. The mixer is followed by a fairly selective bandpass filter to protect the IF amplifiers from first and second adjacent channel signals.

The present invention concerns single balanced mixers, and more 
particularly a single balanced mixer and an output filter for a television 
receiver or the like providing reduced generation of unwanted frequency 
components and improved rejection of adjacent channel frequencies. 
BACKGROUND 
A mixer is a three port network which translates an incoming signal at one 
frequency to some other intermediate frequency. To affect this 
translation, the incoming signal is heterodyned or mixed in a non-linear 
device usually with another signal generated by local oscillator. This 
process generates two primary intermediate frequency signals, having 
frequencies which are equal to the sum and difference of the incoming 
signal frequency and the local oscillator frequency. However, other 
unwanted frequency components and products are also generated as well as 
the mixing of adjacent channel frequencies which also takes place. 
Single or double balanced mixers when used in the tuners of television 
receivers have several advantages over active mixers which often use 
bipolar or MOSFET transistors for the VHF band and a single Schottky diode 
for the UHF band. These advantages include improved channel 6 beat 
performance, improved cross-modulation performance, improved half-IF 
performance, partial cancellation of local oscillator energy at the signal 
input port, and sufficiently wide bandwidth for use of one mixer over a 
plurality of bands, such as UHF and VHF. One of the reasons for these 
advantages of a passive mixer over an active mixer is that for an active 
mixer the unwanted cross-modulation products, intermodulation products and 
harmonics are amplified during the modulation process and it is then too 
late to do much beneficial filtering. 
In many television receivers the IF stages comprise a surface acoustic wave 
(SAW) filter which provides excellent selectivity without requiring the 
alignment of coils but has a large insertion loss of the order of 20 db. 
Accordingly, it is often the practice to provide about 20-26 db 
amplification prior to the SAW filters to make up for this insertion loss 
in order to have the level of the output signal from the SAW be no lower 
than the signal output level from the mixer in order to maintain a 
satisfactory signal to noise ratio. Since the tuner RF bandpass filter 
circuits have relatively wider bandwidth than a single channel, 
substantial adjacent channel signal is present in the output of the mixer 
and is fed to the SAW preamplifier. 
This adjacent channel signal causes increased cross modulation and 
intermodulation products in the mixer which are aggrevated in an active 
mixer. Additionally, this adjacent channel signal when applied to the SAW 
preamp can overload the SAW preamp or at least adds sufficient extraneous 
signal level to drive the SAW preamp through a larger dynamic response 
characteristic and thus increase the distortion of the preamp. However, 
even if such a preamp is not used, it is still desirable to protect the IF 
stages from this extraneous signal for reducing the dynamic range of the 
IF stages and thus reduce the distortion products. 
The adjacent channel signals present more of a problem for the VHF 
television channels of channels 2-13 than for the UHF channels primarily 
because the UHF stations broadcast with at least 6 channels of separation 
in any particular market area. In such a case, even though the UHF tuners 
use a single Schottky diode as a mixer with a following bandpass filter, 
such tuners are not balanced and thus do not provide the lower 
cross-modulation, intermodulation, and harmonic products achievable with a 
balanced mixer in the VHF band. 
Accordingly, it is desirable to provide a passive mixer such as a single or 
double balanced mixer for a television receiver wherein the unwanted 
frequency components, harmonics of the fundamental signals, and distortion 
products conveyed to the IF stages are reduced. 
As used herein, the term television receiver includes television processing 
apparatus without limitation as to the presence or absence of a video 
display, e.g., a television set, a VCR, etc. 
SUMMARY OF THE INVENTION 
The present invention concerns a mixer and output filter used in the tuner 
of a television receiver or the like. The mixer is a single balanced mixer 
for producing an intermediate frequency output signal from an input RF 
signal and a local oscillator signal. A fairly selective bandpass filter 
in addition to the usual IF filter is coupled to the output of the mixer 
for filtering the output signal in order to protect the IF amplifiers from 
first and second adjacent channel signals and undesirable distortion 
products.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings wherein like reference numerals have been 
applied to like members, FIG. 1 shows a single or double balanced mixer 
14, receiving RF signals received at antenna 10 which are coupled to an RF 
amplifier 12 in which the RF signal corresponding to the selected channel 
is selected. A local oscillator 16 generating a signal having a frequency 
corresponding to the selected channel is coupled to mixer 14. Mixer 14 
combines the selected RF signal and the local oscillator signal to produce 
an output signal having sum and difference frequency components. The 
output signal of mixer 14 is coupled to first bandpass filter 18 which in 
turn is subsequently coupled to an IF section or second bandpass filter 
22, through a buffer or amplifier 20. The buffer 20 is optional, and when 
used can provide isolation, impedance matching between first filter 18 and 
second filter 22 and/or signal gain particularly if a surface acoustic 
wave (SAW) filter is used in the IF section. 
IF section 22 is a standard IF stage which can include SAW filters, single 
or double tuned filter sections, and amplifiers. The output of filter 22 
is fed at 23 to appropriate detectors (not shown) for video and sound 
demodulation. 
The frequency of the local oscillator signal for each channel is controlled 
so that, as in the exemplary embodiment, the difference component is in 
the passband of the subsequent IF section 22 which, for example, in the 
United States is approximately between 41 and 46 MHz and typically has 
sound and picture carrier frequencies at 41.25 MHz and 45.75 MHz, 
respectively. 
Referring now to FIG. 2, there is shown a graph of the amplitude vs. 
frequency response of the first bandpass filter 18. Marker "a" is the 
crest of the response curve at the center of the IF passband and is at a 0 
db reference level. Marker "b" is the picture carrier at 45.75 MHz and is 
down 1 db, marker "c" is the chroma sub-carrier at 42.17 MHz and is down 1 
db, marker "d" is the sound carrier at 41.25 MHz and is at a level of -3 
db, marker "e" at 39.75 MHz is the first adjacent picture carrier at -9 
db, marker "f" at 47.25 MHz is the first adjacent sound carrier at -6.35 
db, marker "g" at 51.7 MHz is the second adjacent picture carrier at -19.8 
db, and marker "h" at 35.25 MHz is the second adjacent sound carrier at 
-21.9 db. 
The RF amplifier 12 and mixer 14 have essentially a flat response over this 
frequency range. Thus, as shown in FIG. 2, the first and second adjacent 
picture and sound carriers are substantially rejected and not passed onto 
the IF section or second bandpass filter 22 for the reasons and beneficial 
effects discussed hereinabove. 
Referring now to FIG. 3, there is shown an exemplary tuner for a television 
set, FM radio receiver, or the like. For an exemplary range of 
frequencies, e.g., including the VHF and UHF bands, the RF input signal is 
derived from an antenna 300 or any equivalent source. The signal from 
antenna 300 is then amplified in RF amplifier 301 which can be any 
amplifier common in the art. The output of amplifier 301 is fed to a tank 
circuit 302 having a primary coil 303 tuned with a primary capacitor 304, 
e.g., a varactor diode, and a secondary coil 305 tuned with a secondary 
capacitor 306, e.g., also a varactor diode with an appropriate mutual 
coupling between coils 303 and 305. The output from the tank circuit 302 
is then fed through impedance matching inductance 307 to RF input 314 
coupled to secondary winding 328 of transformer 324 comprising a balanced 
input of mixer 310 which corresponds to mixer 14 of FIG. 1. A local 
oscillator signal is coupled to the primary 326 of transformer 324 of 
mixer 310. 
Mixer 310 is a single balanced mixer having a pair of diodes 320 and 322 as 
the mixing elements and a transformer 324 having a primary winding 326 and 
a secondary winding 328 having a center tap 330. Transformer 324 can be 
either a closely coupled RF transformer well known in the art or 
alternatively, a balun having, e.g., a 50 ohm unbalanced or single ended 
input and a balanced or double-ended 200 ohm secondary with a center tap. 
When transformer 324 is a balun, it contains pairs of bifilar wires wound 
around a ferrite core and connected in a conventional fashion to provide 
the impedance transformation between the unbalanced (single ended) input 
and the balanced (doubled ended) output fed to diodes 320 and 322. Diodes 
320 and 322 are non-linear unidirectional current conducting devices 
connected in series with each other at an output terminal 332 and coupled 
across and poled for unidirectional current conduction between terminals 
36 and 38 of secondary winding 328. In the exemplary embodiment diodes 320 
and 322 are Schottky diodes. The intermediate frequency signal at terminal 
332 is then coupled to bandpass filter 316 which corresponds to first 
bandpass filter 18 of FIG. 1. 
Filter 316 is more selective than the usual IF filters in tuners and 
protects succeeding IF amplifiers from many of the undesired mixer 
products as well as first and second adjacent channels which can cause 
distortion products in subsequent amplifier stages. A double tuned filter, 
which in the exemplary embodiment is a Butterworth filter, having a center 
bandpass frequency of 44 MHz with a bandpass of 45.75 MHz and 42.17 MHz 
has been shown to be suitable. This filter 316 substantially improves the 
harmonic and distortion characteristics of mixer 310. 
The structure of bandpass filter 316 and comprises in the exemplary 
embodiment for television use a double tuned Butterworth filter although 
filters of greater selectivity can be used, e.g., triple tuned. Resistor 
344 of e.g., 330 ohms, is connected from output terminal 332 of mixer 310 
to ground in parallel with capacitor 346, e.g., 330 picofarads (pf). A 
capacitor 348, e.g., 320 picofarads, couples the output signal from 
terminal 332 to a "pi" configuration of inductances 350 and 354, e.g., 
each 120 nanohenries (nhy) and inductance 352, e.g., 10-15 nhy, with 
inductance 352 providing the coupling between inductances 350 and 352. 
Capacitor 356, e.g., 68 pf, couples the output end of inductance 354 to 
ground and the signal is fed through capacitor 358, e.g., 47 pf, to IF 
output terminal 318. 
The filter 316 protects the subsequent IF amplifiers from the first and 
second adjacent channel signals for reducing distortion products in said 
subsequent amplifiers as well as reducing harmonic products in mixer 310 
by maintaining high circuit impedances at the frequencies of the unwanted 
signals. 
While it has been illustrated and described what is at present considered 
to be a preferred embodiment of the present invention, it will be 
appreciated that numerous changes and modifications are likely to occur to 
those skilled in the art and it is intended in the appended claims to 
cover all the changes and modifications which fall within the spirit and 
scope of the present invention.