Mitigating strong AM broadcast interference on direct converison ham receiver

[wb0gaz]

I'm looking for suggestion(s) as to how I can mitigate unwanted RF from a nearby AM broadcast station that may be arriving common-mode style and adversely impacting operation of my (HF/ham radio homebrew) receiver. Based on some observations (below), I think the AM broadcast signal is taking a common-mode path from antenna/wiring to the receiver where it's detected regardless of which frequency the receiver is tuned to.

The receiver is a homebrew version of the "usdx" simple microcontroller-as-DSP HF receiver, which at it's core is basically a direct conversion, phasing detector design followed by gain stages. All (all) of the receiver's gain follows the "tayloe" detector, a 60 dB op amp stage followed by process gain in the embedded DSP code.

There is a nearby, high power AM broadcast station at 1690 kHz - about 2 miles from home. It produces measurable voltage at the port of my lowest-band, longest-wire antenna during daytime, so it is very strong energy source.

The receiver appears to be detecting the AM signal regardless of tuned frequency (as if the 60 dB gain stage is simply operating on rectified audio from the AM broadcast station, along with the (sometimes weaker, sometimes stronger) intended signals coming from the Tayloe detector and associated quadrature clock.

A sharp high-pass cut-off filter (about 2 MHz corner) in the antenna-to-radio (coax feedline) path significantly reduces artifacts from the AM BC station on other (conventional superhet) receivers here, but this receiver is unaffected by presence/absence of the high pass filter (however, if I disconnect coax from the filter to the receiver, the AM detected audio, as well as any intended signals, disappear.)

I think the culprit may be common mode path conducting 1690 kHz into the receiver's circuitry, as the radio is acting like a crystal set (the AM station's detected audio level is unaffected by any of the RF circuitry - mainly RF filters and associated switching - in the receiver.

Would a RF transformer (i.e., two windings sharing a common powdered iron or ferrite core) interrupt a common mode path? Is there any systematic way of identifying the path this 1690 kHz AM signal is taking to get into the receiver?

Thanks for any pointers, suggestions, or requests for clarification/improvement to my question.

[TimFox]

If you think the baseband (modulating) waveform is overloading your front end, it may be due to a rectifying contact in the antenna input section of your device.
This can occur with an oxidized copper connection, since copper oxide is a semiconductor.

[iMo]

.. Would a RF transformer (i.e., two windings sharing a common powdered iron or ferrite core) interrupt a common mode path? Is there any systematic way of identifying the path this 1690 kHz AM signal is taking to get into the receiver? ..

Such a strong AM signal could be detected/demodulated anywhere in your usdx receiver. What I would do - I would put the receiver into a metallic enclosure (for a test), while I would put ferrite beads/toroids on any wires and the antenna coax coming in/out of the box. Blocking the 1.7MHz would require a larger inductance of the common mode chokes, like several hundreds microhenry, thus you have to make a lot of turns. For example with a ferrite toroid with AL=1000nH/t^2 you would need, say, at least 20 turns (400uH).
Also doublecheck grounding paths and decoupling/blocking with ceramic capacitors (~100nF). Do use battery for the test (and put it inside the box).
You may also disconnect the antenna and try to touch the various points in the radio with a small antenna (like 20cm of wire) trying to find the point where it comes to the strongest detection.
I would also doublecheck the wiring around the Tayloe detector - that kind of detector can process large signals well - it could be there is an issue too (wrong wiring, missing blocking capacitors, etc).

[wb0gaz]

imo - thank you very much - many practical and useful ideas!

Much appreciate the wide-ranging view,

Dave

[babysitter]

I was living approx 6 km of Europawelle @1422 kHz, hd to remove a pair of protection diodes from a (Teltow) receiver frontend to supress reception in the short-wave-band. Also, consider applying a step attenuator and see how it behaves - if the ghost signal go down proportional to attenuation increase or differently.

vy 73
dg3hda

[jonpaul]

1. most modern / digital / RX,SA have poor third order IM and image rejection as no pre-filter.

2, use CMEMI choke and LPF matched to antenna Zo

3, Shielded grounded métal, case

4 add VDE X,Y caps at PSU or mains if not present, most Chinese electronics have no mains EMI filtering

Jon

[wb0gaz]

Tks for the ideas --- this is homebrew, so free to modify as needed.

External attenuator - that's a good point, would help me understand the mechanism. At night (the broadcaster goes to lower power level) the problem completely disappears and the receiver works well.

[wb0gaz]

Time for a "reset" on this problem!

Turns out I need a band-stop filter with enough poles that it looks relatively transparent 100 kHz away from it's "notch" frequency (I wish to have it serviceable in the 1.8-2.0 MHz ham band), as there are no other strong, close-by AM broadcast stations below 1690 kHz, and ideally with ability to pass transmit power levels (at least 100W) so I can avoid switching the filter in for receive only.

I suspect this would need to be designed and fabricated from scratch as the spec is very specific to my individual environment.

For more details, see below:

###

I (finally, belatedly) just stuck spectrum analyzer on the antenna feedline, and after adding enough attenuation to the analyzer input, found +10 dBm @ 1690 kHz on the antenna (a wire loop) which is in plane with the broadcast station a couple of miles away.

I also measured 0 dBm at 1690 kHz with feedline going through a "dunestar 400HPF" 160 meter high pass filter (below copied from Dunestar's product page):

SPECS:

    The roll off is very steep below 1.8Mhz. Minimum below 1.6Mhz is 40-db at 1.0Mhz.
    Insertion loss is typically .7-.9db above 1.8Mhz.
    Dimentions: HWD 2.0 x 6.0 x 1.75 inches (including mounting flange)
    Power rating 200 Watts PEP

I've not tried to sweep the filter, but did realize I had erred in assumption that 1690 kHz would be below the transition band, which is not what I've observed (about 10 dB insertion loss at 1690 kHz) nor what the specs (copied from vendor product page) says.

So, this is no complaint about Dunestar 400HPF, but does say that it wasn't intended to solve this specific problem.

My statement in initial posting that this could be a common mode problem is incorrect - helpful postings contribute to learning (thanks!), but were responding to my incorrectly stated problem.

[DaleWentz]

This is the fix for most BCI on QRP rigs. We have built about 6 of these for members in our club and all have been very happy with them.
https://www.wentztech.com/radio/Projects/files/QRP-BCI.html