Swept Front-End Alignment
Most service manuals tell you to align a tuner front-end for maximum signal at two spot frequencies, usually 90 and 106 MHz. You adjust the inductor slugs at the low frequency and the trimmer capacitors at the high frequency, going back and forth until the voltage at a test point is simultaneously maximum at both frequencies. However, this procedure isn't optimum when the front-end has a bandpass response.
This shows the front-end response of a Carver TX-11b tuned to 90 MHz when spot-frequency aligned according to the service manual. The spectrum covers 88 to 108 MHz. The vertical scale is 10 dB/div. You can see a peak right at 90 MHz with the remnants of a bandpass response sloping off higher in frequency. I drove the tuner with an HP 8443A tracking generator and picked up residual RF in the front-end IF output with a Tektronix P6202A active probe feeding an HP 141T/8553B/8552B spectrum analyzer.
This shows the response after I realigned the front-end to flatten and center the RF passband at 90 MHz (and 106 MHz, not shown). Response is much more uniform around the tuned frequency. Performance of a tuner aligned this way will be less susceptible to temperature drift and component aging. An analog tuner will be more robust to mistuning. The bandpass alignment altered the stereo distortion, which I reminimized, but had a negligible effect on sensitivity. The TX-11b has one tuned circuit before the RF amplifier and three between it and the mixer [1-3].
This shows overlapped traces for a Sony ST-S555ES [2-2] tuned to 90, 98, and 106 MHz after swept alignment. The vertical scale is 2 dB/div. This RF passband is narrower, with some tiny response wiggles. I neglected to trim the spectrum analyzer horizontal scale; the responses actually are centered at 90, 98, and 106 MHz. The 98-MHz response isn't quite flat, but the error is less than 1 dB.
This is the response of a Pioneer F-90 [1-2] after swept front-end alignment. Evidently this tuner is not designed to have a flat RF passband. Narrowing the sweep shows a rounded hump that tracks well across the band. (The falling response with increasing frequency is a measurement artifact. Tuner sensitivity actually is 1 dB greater at 106 MHz than at 90 MHz.)
The local oscillator adjustments affect tracking by altering the varactor tuning voltage. Sweeping the front-end makes the effects obvious: the passband simply moves up or down the band. First set the varactor tuning voltage according to the service manual. Next, tune the tuner to 90 MHz and optimize the RF passband shape using the front-end inductors. Then adjust the LO inductor slug to center the passband at 90 MHz. Repeat at 106 MHz using the front-end capacitors and LO trimmer capacitor. Go back and forth until the passband is flat and centered at both frequencies.
More is here.
Updated March 4, 2008
