[1000mp] Inrad Front-end Mod

[George, W5YR]

----- Original Message ----- 
From: "tbeltran" <tbeltran@earthlink.net>
To: <1000mp@mailman.qth.net>
Sent: Monday, January 19, 2004 1:26 AM
Subject: Re: [1000mp] Inrad Front-end Mod


> Thanks to all who responded.
>
> George, I am feeding the 606A with 50 ohms to the input of the Mark V.  Is
> there something more I should do?

Tom, I am not sure exactly what this means. It is required that the output
of the 606A be terminated in a 50 ohm resisitve load  in order for its
output level and attenuation calibration, etc. to be accurate. If it isn't,
the voltage level of its output will not be correctly related to the
metering and the attenuator calibration.

I checked my 606A and with the output unterminated, I set the level to read
0 dB on an HP 411A RF Millivoltmeter that has been calibrated against an HP
436A Power Meter with 0.5% accuracy.

When I placed a 50 ohm load on the 606A output, the output level dropped 6
dB indicating that the voltage had been halved. This verifies the 50 ohm
internal impedance of the 606A, at least at 14.1 MHz.

As a check, and out of curiosity, I terminated the 606A in the antenna input
of my Icom PRO2. With the 606A set for an output of -73 dBm, which
corresponds to 50 microvolts across 50 ohms, the PRO2 S-meter read exactly
S9. I had previously calibrated the S-meter and knew that S9 represented an
antenna input of -73 dBm. So, by implication then the input impedance of the
PRO2 on 20 meters (14.1 MHz) is about 50 ohms resistive.

If your MK V input is also 50 ohms, then whatever you read with the 606A
should be as accurate as the calibration of the 606A. If it is not, then the
error will be proportional to the difference between the actual input
resistance and 50 ohms. You should not  use a 50 ohm resistor either in
series with or in parallel with the 606A output at the MKV input. Any added
resistance will increase the error further.

I should point out that most 606A's tend to be rather loosely calibrated in
the voltageand dBm settings. I always use the external 411A to set the
output level to 0 dBm and the rely upon the accuracy of the attenuator which
is quite good.

But, again, unless the output is terminated in 50 ohms, its output voltage
will not be as stated on the internal meter,  expressed either in rms
voltage or dBm.

73, George W5YR
w5yr@att.net


>
> And Ron, thanks for the math - that is way beyond me.
>
> Let me add that I set my signal gen. to CW, and I tuned it to 14.1 (I used
> 20 meters only because that is the band where whether using the tuned or
> flat RF preamp position, it is the same - flat.) I then tuned in the
signal
> like any CW signal, using the indicator on the Mark V.
>
>
> >The service manual has you set the IF gain using only 1 block on the
> >s-meter lit with a +11dbu source.
>
> I wonder what this would be in microvolts - I'd like to check it out and
see
> if it verifies my current setting.  Harry's setting of 10 is more like
it -
> I think Inrad says to reduce the gain by 1 or 2 points.  Next week, I'm
> going to take my rig to a friend's house - he has a calibrated signal
> generator and other tools - and more importantly, he knows how to use
them.
>
> This whole thing is quite interesting to me - compared to my Drake R4C
(with
> Sherwood mods), my Mark V always seemed noisy to me.  Right now at 4, it
> seems much improved, but, it may be too light as Harry found out.  Tom
W6EIJ