Tuning Input Cavities on HPT Series Receivers – FM Band
Warning: Do not attempt to retune the cavity using only a single frequency and tuning for maximum output. The units may only be properly tuned using a spectrum analyzer and tracking generator.
The STL Receivers have a 10 MHz wide bandwidth and will generally not require retuning when operated between 944 to 952 MHz.
The 88 to 108 MHz CRX module is the interface between the antenna and the converter or mixer circuit. The function of this circuit is to pre-select the desire bandwidth of signal and amplify the signal with low noise and intermodulation products.
The module consists of a four cell filter followed by a wideband amplifier with up to 25 dB of gain. The band pass is approximately 1 MHz, so any large change in frequency will require re-tuning of the module.
RF Antenna Amplifier Calibration:
Connect the tracking generator (-30dBm output) to the input of the receiver. Disconnect the BNC connector from the output of the module and run a short 50 ohm cable to the input of the spectrum analyzer. Adjust the cells so as to obtain the maximum gain of 20 to 25dB with 8MHz bandwidth plus and minus 1 dB. Verify that the response curve is at least 60 dB down at 20 MHz from the center frequency.
If available, place an SWR bridge between the tracking generator and adjust the input variable capacitor for 15db return loss or better.
Sample Response when correctly tuned: