Regarding my previous inquiry, Thanks all for your thoughtful replies:
roger1818 - My guess (and it is only a guess) is that the switch would:...
Your guess is very close to what effectively seems to be occurring, at least in the CM-7777, although it appears that no amplifiers are actually switched off.
Jase88 - "Combined or separate" pre-amps have two antenna inputs: VHF, and UHF or combined.
Jase88, your description is is pretty much operationally correct, although doesn't address what is actually happening internally.
balm - Mine doesn't say this
Well, not exactly sure what mine said either, since the instructions are stored away elsewhere (a few hundred miles from my current location). But, the CM-0264DSB does have both a tunable notch for FM and a FM band-reject filter, whereas the CM-7777 has only the FM band-reject filter. And there were some special instructions regarding the activation of FM rejection circuitry (in addition to tuning the frequency of the notch to match the station to be rejected) for the CM-0264 that was not present in the instructions for the CM-7777.
Ken Nist (KQ6QV) kindly provided a partial schematic of the "Combined/Separate" switching arrangement for the CM-7777:
Ken's partial schematic well supports his, and others, premise that the VHF and UHF signals are strictly processed separately. And for the CM-7777, it also somewhat alleviates my concern about excess thermal noise from the UHF portion being injected into the VHF section. Most likely the filters eliminate such.
Thanks for the help.
A while back intravino asked about the Winegard AP-3700
intravino - Does someone have or had the Winegard VHF/FM AP-3700 Preamp?
Intravino, the AP-3700 (VHF-only preamp) was compared directly against the CM-7777 (using only VHF section, "Separate", UHF input loaded with a 75 Ohm terminator*) for channel 13 reception.
* The 75 Ohm terminator seemed like a good thing to do at the time, because of pre-amplifier stability concerns and, of course, for weather protection. But, still I'm unsure whether it affects noise levels into the combined output or not. This probably depends on how good the internal VHF/UHF filtering is. The thermal noise (kTB) originating and amplified from a resistor source (terminated load on input) is almost certainly greater than noise originating from purely reactive loads (unterminated inputs).
The driving factor for the preamp comparison was a concern that a strong FM station at 89.7 MHz would not be rejected sufficiently by the CM-7777 preamp (without the additional notch that the AP-3700 had). Note: 89.7 MHz is very close to the upper edge of channel 6 at 88 MHz, so it is difficult to suppress 89.7 MHz without also affecting channel 6. Guess suppression of FM signals at the lower end of the FM band vs gain on TV channel 6, or maybe even channel 5, is a tradeoff that is made by the designers of the FM band-reject filters that are used in various preamps.
Before installation, a system noise figure analysis was performed. The two preamps provided very close to the same system noise figures. If the CM-7000 (DTV converter used in the comparison test) was assumed to have a greater than ~8 DB noise figure, then the CM-7777 had a small advantage. If the CM-7000 had a noise figure less than about 5 dB, then the AP-3700 gained the advantage.
Specs for the two preamps:
AP-3700: Gain=17 dB; Noise Figure= 2.6 dB
CM-7777(VHF): Gain= 23dB; Noise Figure= 2.8 dB
Test conditions were:
- Pair of vertically stacked Winegard YA-1713 Yagi at 62 feet above ground level
- Both preamps located between the stacked antennas (about 4 feet RG-6 connecting each antenna through a combiner, Winegard CC 7870)
- Transmission line 85 feet (RG-6)
- Receiver front end, Channel Master CM-7000
- Strongest undesired signals:
* FM station at 89.7 MHz (~ -24.5 dBm, ref. FM Fool), and a
* UHF station (~-37.8 dBm, ref. TV Fool)
- Desired signal: channel 13 at -101.4 dBm (ref. TV Fool)
- Both preamps were tested for about five to ten minutes each, then the tests were repeated.
The indications used for comparison:
CM-7000 signal goodness indicator (probably a bit error rate (BER) indication), plus picture quality. The picture quality remained visually near perfect even with low numbers on the CM-7000 indication.
The stacked antennas were rotated a fixed distance in azimuth for each test, to ensure less than 100% indications. When the antenna was pointed at bore sight (dead-on), the signal 'goodness' indicated on CM-7000 was close to 100%, sometimes slightly less.
The monitoring was accomplished with the help of two assistants, one at the TV watching and announcing CM-7000 indications, another relaying CM-7000 readings (approximately every second or when the numbers changed) to person at top of tower.
A Channel Master signal level meter (similar to Sadelco FSM-3 or FSM-4) was available for other measurements (such as optimizing stacking distance or spacing, and determining bore sight/center beamwidth direction). The signal level meter was not used to directly compare preamps. The signal level meter doesn't directly help in comparing signal/noise ratios (S/N).
Tests for signal overload were performed by inserting and removing a Pico Macom HLSJ (VHF hi-low splitter/combiner). The HLSJ, in combination with a 75 Ohm terminator on the unused port, can be used as an excellent FM reject filter.
Not being sure about the CM-7000 response to FM signals, there was also a concern about overload of the CM-7000 from the FM signal or any other signal that may have been be simultaneously presented to the CM-7000 input. Tests for CM-7000 overload were performed by insertion of attenuation at the CM-7000 input and looking for changes in signal "goodness' on channel 13. No indications of CM-7000 overload were observed.
If there had been any indication of overload, it would have probably occurred with the CM-7777. During setup it was confirmed that FM rejection of the the AP-3700 (at the notch frequency) was much superior to the CM-7777. Plus the lower gain of the AP-3700 may have helped with system dynamic range.
There was virtually no difference in the performance of the two preamps.
In the end, for no particular reason, the CM-7777 was selected.