Swept Front-End Alignment

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 may not be 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 horizontal span is 88108 MHz. The vertical scale is 10 dB/div. The response peak is right at 90 MHz, but you can see the remnants of a bandpass response sloping off higher in frequency. Smaller pips are local broadcast signals leaking into the test setup, and the large pip to the right is the local oscillator. I drove the tuner with an HP 8443A tracking generator and sampled residual RF in the front-end IF output with a Tektronix P6202A active probe feeding an HP 141T/8553B/8552B spectrum analyzer.

This is the response after I realigned the front-end to flatten and center the RF passband at 90 and 106 MHz. 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 better tolerate mistuning. The bandpass alignment altered the stereo distortion, which I reminimized, but it 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 is the response of a Sony ST-S555ES [2-2] after swept alignment. The passband is a bit narrower than that of the TX-11b, but the skirt response is nearly identical. You adjust the coils in this tuner by squeezing turns.

This is a Pioneer F-90 [1-2] after swept alignment. The skirt response is like that of the other tuners, but the passband is peaked instead of flat. There is a slight kink in the slope near the peak, but no adjustment would yield a flat top. Swept alignment offers little advantage for this tuner, although you can't tell until you try it.

The local oscillator adjustments affect tracking by altering the varactor tuning voltage. Sweeping the front-end reveals their effect: the passband simply moves up or down in frequency without changing shape. Therefore, align the front-end as follows. 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 and LO trimmer capacitors (adjust the varactor tuning voltage if the LO has no trimmer capacitor). Go back and forth until the passband is flat and centered at both frequencies. Finally, check the response at 98 MHz.

March 31, 201188108 MHz