Radiation from the RX Ensemble II
- Hi all,
I first want to thank everyone for their contributions to the success of the Softrocks.
I enjoyed building the RX Ensemble II and with the great building instructions on wb5rvz.com building was without problems.
There was one weird thing I noticed during testing the band filter switching. I had a DMM (Peaktech 2010) hooked to the receiver 5V. When the frequency was set to above e.g. 20MHz the reading on the DMM dropped from 5V to 4.8V After checking with an analog meter without semiconductors it turned out that the 5V was stable.
So the conclusion was that the DMM was disturbed by a HF voltage.
I checked the cables for HF common mode currents by attaching a VNWA in spectrum analyser mode via a one turn current transformer on a ferrite suppression clamp core. It turned out that the Si570 frequency (4xLO) was present on the cables.
Inspection of the circuit diagram and the PCB layout revealed that the polarity/phase of the Si570 output transformer (T1) is ok, but if T1 is inserted with straight leads into the PCB the primary and secondary windings will have opposite phase. The result is that a common mode voltage is created via T1. It was easy to silence a FM broadcast radio at 1m distance!
Swapping both leads of the secondary winding reduced the common mode current by 20dB. A similar result was obtained by directly connecting the USB ground to receiver ground, but a lot of spurs popped up in the receiver spectrum. A 10nF capacitor gave roughly the same reduction without the spurs.
So did I overlook something or might an addition to the building instructions be useful?
Just my 2 cents and sorry for the bandwidth.
73 Gerhard PA1GVE
- But this discussion relates to radiation from the connecting cables.
----- Original Message -----
From: "Peter" <pete@...>
Sent: Thursday, July 19, 2012 1:26 PM
Subject: [softrock40] Re: Radiation from the RX Ensemble II
> Many years ago I was involved in studying the "Zero-IF" receiver architecture (also called "direct conversion", "synchrodyne" and
> "homodyne" for a military VHF application. Direct conversion ham receivers like the Neophyte common at the time, so I was struck
> by one item that turned up in the study: Zero-IF receivers for critical applications almost always included some kind of RF
> amplifier between the antenna and the mixer, even if gain wasn't needed. The amplifier provided reverse isolation, greatly
> reducing LO radiation. For that reason and others I began including an amplifier ahead of the mixer in the Lazy Dog receivers.