MASTR II local oscillator injection

GE MASTR II receiver sensitivity
as a result of local oscillator injection
Problem and Fix
By Kevin Custer W3KKC

The GE MASTR II high-band and UHF receivers are highly sensitive and selective single conversion receivers. The bandwidth and selectivity are determined by the rf preselector coils, the I-F (intermediate frequency) crystal filters, and partially, the discriminator. Signals received at the antenna input are routed through the rf preselector cavity to the mixer stage to be heterodyned with the injection frequency signal; which is derived by multiplication of the crystal in the ICOM. The resulting I-F signal is then amplified by the 1st I-F amplifier then routed through the I-F crystal filters, more amplification, and more filtering. The output of the I-F stages is limited, and this signal is demodulated, (turned into basic audio), by the discriminator.

The term "single conversion" means the intended receive signal is only converted one time - to another frequency. Most narrow band frequency modulated (NBFM) receivers are dual, or multi - conversion designs, meaning the desired receive frequency is converted to another, and another, and so on. An example is the Hamtronics R100. The R100 is a dual - conversion design. The intended receive frequency is converted to 10.7 MHz. and then converted again to 455 kHz. The main advantage of dual conversion, is the final selectivity is achieved by use of a highly selective (however less costly) 455 kHz monolithic I-F filter. GE designed the MASTR II with only higher frequency (11.2 MHz) crystal filters for its selectivity.

As mentioned above, the desired receive frequency is converted in the mixer to the I-F frequency by a process known as "heterodyning". Heterodyning is achieved by injecting two frequencies into a non linear amplifier, (mixer) to create another frequency (the I-F frequency). The mixer amplifier has to be non linear in order to create the I-F frequency, otherwise the two frequencies (the desired rf carrier frequency, and the injection frequency) would simply be amplified and no other frequency would result. The injection frequency is created by the local oscillator (channel ICOM) that is multiplied up to the desired frequency - - plus or minus the I-F frequency. That's 11.2 MHz in the case of the MASTR II VHF and UHF.

Since the mixer would accept - and convert - a receive frequency that is 11.2 MHz above or 11.2 MHz below the injection frequency, a means of selecting which "image" must be provided. If no selection is afforded, the receiver will copy signals on both frequencies equally. The preselector provides this function of selecting the desired image. In quality fm receivers like the MASTR II, a "front end" preselector is used ahead of the mixer to only allow the desired frequency to be processed. The design characteristics of the helical front end determines how well the receiver will deal with the undesired image and out of band signals. In cheap receivers, with little front-end preselection, strong out of band signals, or adjacent in band signals, or both, can overload the rf preamplifier or mixer producing multiple undesired signals to be demodulated. This normally sounds like birdies, squeals, and annoying whistles on top of what you are intending to hear. This is called intermodulation distortion, or simply "intermod".

As stated earlier, the injection frequency is created by the multiplication of the local oscillator, (LO) crystal, (located in the channel ICOM). The MASTR II crystal is multiplied nine times to produce the injection frequency for the mixer. If the injection frequency is above the desired receive frequency, you have high side injection, if the injection frequency is below the desired frequency, you have low side injection.

Because there were more MASTR II radios produced for commercial services, there is a greater surplus of these radios in the commercial bands. Hams take advantage of this surplus, and save money building repeaters and link radios for the ham bands out of such resources.

Commonly available 150.8 - 174 MHz and 450 - 470 MHz receivers were designed to utilize low-side local oscillator injection. Crystal manufacturers will automatically cut a crystal for the ham band for low-side injection, because receivers that are factory made for the ham band are also low side injection. The receivers for the commercial bands are - however- made differently as compared to the ones made for the ham frequencies. This is because the local oscillator and subsequent multiplier chain cannot tune the entire range from 136 to 174 or 406 to 470 without different components. This is why there are frequency ranges of these receivers - the component values are selected to give a certain level of performance. While you can install a ham crystal into the ICOM cut for low side injection, a decrease in performance generally results. This is because the local oscillator and multiplier chain is being operated outside of its intended frequency range.

If - however - you have a ham crystal made for high-side injection, the resulting oscillator and multiplier chain is operating at frequencies that resemble the originl commercial frequencies. The receiver doesn't care which side the injection falls on, and excellent receiver performance results from a commercial receiver operating in the ham band without the need of conversion of the local oscillator and multiplier chain.

This 'trick' works on high-band and UHF commercial receivers being operated in the ham band. It resolves the "self quieting" problem, and modifications to the VHF LO Multiplier are not necessary.

I hope this helps you to understand the concept of high and low side injection, and why you see reference to the injection choice in many articles on Repeater Builder™.

The information presented in and on these conversion pages is © Copyrighted 1995-present by Kevin Custer W3KKC and multiple originating authors.