MORE INFO ON THE DC RCVR AND BODNAR UNIT AS LO

[David Eckhardt]

Further, if ground loops or whatever else were the source of the noise, it would also appear using the HP86408B.  As you saw in my original write-up, the noise is not present using the HP sig. gen.  Only noisy using the Bodnar unit.  I also believe my individual noise measurement of the Bodnar unit, alone, confirms it is the source of the noise and not something with the DC receiver or gound loops.  

When Terry laid out the DC receiver for SM, he was extremely careful to address everything that could possibly go wrong with a DC receiver.  The input port to the DMB is set properly at 50-ohms with a resistance input network.  The IF port of the DBM feeds a constant-impedance termination network.  We also included an audio BPF on the final output of the receiver with the corner set at 15 kHz.  Since the output of the Bodnar unit is zero-to-+13 dBm squarewave, I've also DC blocked that input to the DC LO input so no DC accumulation can take place.  Again, the noise occurs with the DC receiver ONLY with the Bodnar unit and not fed with a clean source from the HP unit.  

I finally did email Leo both write-ups earlier today. He has responded with a request for a screen capture of his GPSDO setup.  I sent that off and have heard no more from his end.  I believe he is investigating using the settings that my unit chose for an LO set to 14.666000 MHz (entry to the unit is in Hz).  As such, Spectrum Lab is set for a center of 4.000.... kHz.  

What's even more puzzling is only with the Bodnar unit is the "center" spike present.  Using the HP source, that center spike at 4 (or 5 if I choose the LO at 14.665000) kHz is not present.  That must be sourced within the Bodnar unit itself.  

Why am I using a DC receiver?  It can address any frequency within the limits of the components based only on the input filter and the LO frequency.  Even the Icom 8600, 7300, or 7610 are not stable enough by themselves to support 1 to 10 Hz doppler.  Sure, I could feed the 10 MHz output from another GPSDO'ed source which I have, but I need to assess that noise output (maybe desperately after this exercise?!) before feeding that to those receivers to establish a stable reference frequency for Doppler.  One last reason for my using a DC receiver:  It's simple and understood with minimal circuitry.  All three of the Icom radios are EXTREMELY complex.  If something doesn't look proper, I'd hate to troubleshoot a highly complex receiving system!   

Dave - WØLEV

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Dave - WØLEV

[David Eckhardt]

I wish I could, but on either the 7610 or 7300, one can not turn off the AGC.  This is a requirement of the direct conversion receivers as the AGC is embedded in the receiver chain to prevent ADC saturation.

If you have found a way to turn off the AGC on the 7610, please email us in this group.  I just checked.  The only settings are "SLOW, MEDIUM, and FAST".  No option to turn the AGC off.

Dave - WØLEV

On Tue, Apr 7, 2026 at 11:31 PM David McClain via groups.io <dbm=refined-audi...@groups.io> wrote:

FWIW… I use an IC-7610 locked to a SRS FS-740 GPSD-OCXO. 

The only way to ascertain the stability of the master clock is by measuring against a more accurate standard, like a Cesium clock. I surmise, from the manual, that I have ≈ 100 μHz accuracy.

But I have really good repeatability in measuring injected reference carriers locked to the same master clock in my lab, where for reference levels with SNR > 60 dB leads to repeatable measurements of IC-7610 PLL tuning offset below 50 μHz at 15 MHz, over periods of several minutes to hours.

As long as you are careful not to overload the ADC in the IC-7610 I get really clean signals. Overload will produce mirror symmetric sidebands, which is a sure sign. I run with no AGC on the IC-7610, but a fair amount of Input Atten (12 dB) and backed-off RF Gain.

I do understand your concerns about the circuit complexity of the IC-7610, but I have pretty good luck with it. In contrast, my trusty old IC-7700 had a habit of inserting an extra DAC sample every hour on the hour. I have not run into any problems with the IC-7610 apart from overload. 

And I do not use the USB Audio channel, but rather take the output from the analog output in the DIN connector on the back of the IC-7610. I take the analog audio and feed into a MOTU-828 phase locked to a WORD CLOCK at 48 kHz, also slaved to the lab frequency reference.

Using a dual RF reference injection at ±1400.5 Hz allows me to determine both the PLL tuning offset, and any error in the ADC clocking.

- de Dave, N7AIG

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Dave - WØLEV

[David Eckhardt]

If this is in one or both of the manuals, I missed it.  So just holding the AGC button down for a period of time turns off the AGC?  I'll have to try that.  But most of the signals we deal with in radio astronomy - my reason for turning it off - are pretty darn weak!  I've used it with AGC as that doesn't seem to "bite" at such low levels.

Thank you for the hint.  I'll have to try it.  

I just got up out of my chair and gave it a try.  No Go.  In SSB, if I hold down the soft button on my 7610 and all I get is a matrix containing the time constants.  I repeated the try several times, but can not turn it off.  Sure, I can set it to 0.1 seconds, but can not disable it.

Dave - WØLEV

On Wed, Apr 8, 2026 at 1:40 PM David McClain via groups.io <dbm=refined-audi...@groups.io> wrote:

Actually, you *can* turn off AGC on the IC-7610. 

I once thought the same as you, but then Chat-GPT provided a hint about turning off the AGC on the 7610. 

OFF is actually one of the choices when you hold down the AGC button for setting.

On Apr 7, 2026, at 18:13, W0LEV via groups.io <davearea51a=gmai...@groups.io> wrote:

I wish I could, but on either the 7610 or 7300, one can not turn off the AGC.  This is a requirement of the direct conversion receivers as the AGC is embedded in the receiver chain to prevent ADC saturation.

If you have found a way to turn off the AGC on the 7610, please email us in this group.  I just checked.  The only settings are "SLOW, MEDIUM, and FAST".  No option to turn the AGC off.

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Dave - WØLEV

[David G. McGaw]

Could you please not use a blue font.  While it is attractive, blue is a difficult color for the eye to focus on.  Thank you.

73,

David N1HAC

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