General Dynamics R2670B & Aeroflex 3550-R Spurious Emissions

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Wiregeek
Posts: 61
Joined: Thu Nov 06, 2014 9:53 am

General Dynamics R2670B & Aeroflex 3550-R Spurious Emissions

Post by Wiregeek »

What a ride! I apologize in advance for the length on this post, but I wanted to share the whole saga of how I came to understand how important equipment maintenance is.

Just got the final wraps on a problem I encountered recently. The short form is that a customer CM300d came in for a PMI and had very poor RX performance on 159.9975 simplex. The solid unlock was occurring at -100 and there was a horrible noise until ~-80, which sounded like heterodyning of the PL tone.

That radio went to Depot three times, with no change in behavior. I'm livid at this point, and I snatched out a VHF XTS2500, threw the simplex frequencies into it, and real-world tested the mobile. It worked flawlessly. Gorgeous Moto Audio every time I keyed up the XTS. I couldn't get it to act stupid.

So I continued digging, and happened to have the XTS on and sitting between the mobile and the Gen Dynamics. I pushed some RF out of the GD and the XTS unlocked - and had the same noise!

Now I'm interested.

I fired up an Aeroflex 3550-R out my my field kit and got nearly identical results. I concluded from that result that the CM300d was in fact faulty in RX at 159.9975 - even after three depot trips. But I'd forgotten about the XTS...

During all of this, my fellow tech was puttering around with an HP 8920A that we had found during inventory. This morning I snagged out the HP and started looking at my Gen Dynamics test set with it. My goal was to run the HP through its paces and verify basic operation before asking to send it out for refurb and cal (or to buy it out of inventory for myself, then send it out for refub and cal :D ).

The GD was still set on 159.9975, so I set the HP to spec an on that frequency, and I saw a carrier. I verified that the GD was set to -120 on the generator, and even threw a load on the RF I/O port, with no change in the carrier on the HP.

Made sure all the radios we had in the shop with that channel were turned off (and the flourescent lights, shop stereo, phone chargers, radio chargers, etc, etc).

Flipped the power switch on the GD and the carrier went away. The Aeroflex went on the bench next and it had a very similar carrier - though a lot less amplitude. The GD broadcasts its spurious carrier out the RF I/O port, while the Aeroflex appeared not to do so out of it's T/R port. I snatched the CM300d from our shipping guy and ran it against the Aeroflex, with as much seperation as I could get out of a 3m test cable, and the radio worked flawlessly. I unlocked at -122 and while I had static, there was no heterodyne-ey noise present. I moved the Aeroflex closer to the radio, and the noise came back.

I OKed the radio and tossed it back to shipping, then threw the GD on the HP again. The GD has spurs at 130.0, 140.0, 150.0, 160.0, 170.0, 180.0 all at various strengths. Cal date 8/2014, expired 8/2015. You can see the spurious emissions internally by using the unit's built-in spec an. the 10mhz commonality makes me think the oscillator is heterodyning with another leaking component, and I believe the root cause is a loose/insufficient ground.

What's next? If this was my personal unit, it'd be spread out on the bench getting a good visual exam with an eye towards replacing caps and fixing grounds right about now... Anyone else ran into this or similar?
GlennD
Posts: 482
Joined: Tue Jun 15, 2004 8:00 am

Re: General Dynamics R2670B & Aeroflex 3550-R Spurious Emissions

Post by GlennD »

Many older monitors depended on the receiver front end to reject spurious signals. Ever look at a CE3? This approach fails on software defined radios with wideband front ends since they can be overwhelmed.
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