[TenTec] Relative losses of 9 MHz option filters in Omni VI

[Ken Brown]

Hi again Jack,

Okay, since you also have the 1.8 kHz filter to experiment with, there 
are a number of things you can do to isolate the problem to fewer 
possible causes. You could compare the two optional filter slots to see 
how the 218 performs in one slot relative to the other.

Judging by the measurements you have already made, I would guess that 
the filter "slot" is not at fault. However I would not yet conclude that 
there is anything wrong with your 217 filter.

The losses for the optional filters available (once upon a time) from 
Ten-Tec for the Omni VI range from a minimum of 2.0 dB for the 218 1.8 
kHz BW filter to as much as 14 dB for the 219 and 221 filters which are 
both 250 Hz BW. The purpose of the amplifier (Q4) and the attenuator, 
which can be put in line with the jumper, is to compensate for the loss 
of the optional filter. The amplifier is always in line with either of 
the narrow optional filters, whenever you use them. The jumper only 
selects whether you also use the attenuator, so you have two net gain 
options. Running the optional filter without the amplifier is not an option.

None of my Ten-Tec manuals specify what the gain of the amplifier is or 
what the loss of the attenuator is. Doing some rough calculations from 
the resistor values in the attenuator, it appears to be about a 6 dB 
attenuator. My guess is that the amplifier is intended to produce 
roughly 12 dB gain. That way with the lossiest filters (the 219 or 221) 
and the attenuator out, the net loss would be only 2 dB. For the 216 and 
217 filters which have respectively 8 dB and 7.5 dB loss, and the 
attenuator in, there would be a net loss of  2 or 1.5 dB. For the least 
lossy optional filters, the 218 and the 220, which have 2.0 and 2.2 dB 
loss, you would have the attenuator in, resulting in a net gain 4 or 3.8 
dB (gain of amplifier, minus loss of attenuator, minus loss of filter 
equals net gain or loss). Your test with the 218 and a net gain of 1/2 S 
unit is consistent with my calculations, to within a precision of 1 dB, 
since 1/2 S unit is 3 dB. Since I am guessing at the amplifier gain, 
that is about as precise as I would expect.

With the 217, and the attenuator in (the way the Ten-Tec instructions 
for the filter recommend), I would expect a net loss about 1.5 dB, or a 
quarter of an S unit. The 3 S units of attenuation you are seeing does 
seem like way, way too much loss through the filter. HOWEVER you must 
remember that the filter is narrow, and the peak response of the default 
2.4 kHz BW filter and the optional filter are not at exactly the same 
frequency. You need to tune for peak S meter reading with the 217 in, 
take a reading, and then switch to no optional filters and retune for 
the peak on the S meter and take another reading. If you do not tune for 
the peaks, you are not comparing the right readings.

It is also important that the 6.3 MHz IF bandpass is coincident with the 
9 MHz bandpass. To make this adjustment less critical, choose the 2.4 
kHz filter in the 6.3 MHz IF. On an option 3 Omni VI (or an Omni VI +) 
this is when you have neither 1.8, 500 or 250 selected. The PBT also has 
to be adjusted for peak signal strength. Only when you are doing all of 
the above, you are really using the right S meter readings to confirm 
the relative losses of the filters.

If you make the measurements the way I have described, and you still get 
two or three S units of attenuation with the 217 filter, relative to no 
optional filter selected, then the 217 is probably faulty. In that case 
the first thing I would do is re-flow all the solder connections on the 
filter. If that doesn't fix it, it may have a bad crystal, or capacitor.

Let me know what you find. I hope my explanation helps.

73 DE N6KB


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