Eliminating Front-End Overload with RF AGC
A Kenwood L-07TII that recently came my way had the highest sensitivity of any tuner I've ever measured. But with an outdoor antenna it showed a fatal flaw: background noise on strong signals, and no hint of weak signals that other tuners received clearly. These symptoms suggested front-end overload.
This is the mixer output spectrum with the tuner driven by two 79-dBf test signals 200 kHz apart. Very strong intermodulation products are visible across the 2-MHz passband.
An RF-gain control can solve this problem. With it you can dial back the front-end gain until a signal clears. But there was no rotary control available on the tuner's front panel, and I didn't want to drill a hole. I decided to see if automatic gain control was feasible.
The tuner uses cascaded dual-gate MOSFET RF stages. Nonsignal gate G2 is biased at +7 volts in the amplifiers, which have identical circuits. Lowering the G2 voltage drops the stage gain considerably.
A separate IF strip drives the signal-strength meter, with the first amplifier stage feeding a pair of ceramic filters. I wanted the AGC to respond to the wideband mixer signal so I fed the AGC detector from this amplifier. The ceramic filter input impedance varies somewhat across the passband, but it doesn't affect the signal enough to warrant building a separate AGC amplifier.
This is the AGC detector circuit. The 2N3904 collector connects to G2 in each RF stage, pulling the gates toward ground as the signal level increases. Gain reduction does not begin until the input signal reaches 52 dBf. The dynamic range is enough to handle any signal on the air. G2 is bypassed in each amplifier and I saw no instability when paralleling the gates.
This shows the mixer output with AGC, with the spectrum analyzer sensitivity readjusted to yield the same peak level. All intermodulation products are gone except for two tiny pips about 57 dB down. This was the worst case I could find.
The AGC dramatically improved reception in my high-RF location.
Kenwood KT-880D
A Kenwood KT-880D exhibited front-end overload with my antenna pointed toward a 14.5-kW station 2.4 miles away. This orientation yields 123 dBf, or about 400 mV, at the tuner—a tremendous signal level. Several stations within a couple MHz had clear signs of desensitization or IMD. The KT-880D uses 3SK122 dual-gate MOSFETs in the RF amplifier and mixer. G2 is biased at 3 volts in the RF amp. Grounding G2 dropped the front-end gain 22 dB. This slightly lowered IMD on one or two stations, but was not nearly as effective as inserting a variable RF attenuator in the feedline. The attenuator cleaned up every signal if I dialed in enough loss (nearly 20 dB was required in some cases).Many FETs become less linear when biased for lower gain. Although I saw no sign of this problem in the L-07TII, the KT-880D seems to be a different story. I've seen the 3SK122 used in several tuners but never with RF AGC. Perhaps now I know why.
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Updated July 8, 2007
