RRC Switching Regulator Noise

[g4swx]

I have been using a number of RRC128 MkII for several years with great success, including a 144MHz moonbounce demonstration station as GB2EME.

However I have always been aware of some low level noise around both the control and radio RRC1258 boxes. As I have to run another remote 144MHz EME demonstration in a month or so I thought I would check things out. After having ensured that all Ethernet connections in the shack were screened and all power supplies were linear I still could observe a small increase in the noise floor from the direction of my shack. Switching off the RRC1258 eliminated the noise! After removing the Ethernet connection careful checks with firstly a FT817 portable RX and then with a spectrum analyser with a loop pick-up showed that the noise source was the RRC1258 12V power supply lead. Changing the power supply demonstrated that the source was indeed the RRC1258. The noise is identical in both radio and control RRCs (I have several units and all are the same).

The noise has a 50MHz wide peak centred around 150MHz and consists of a comb of unstable peaks with a spacing of around 6-800KHz. This spacing varies with changing the applied voltage around 12V.
It seems to be coming from the switching regulator within the RRC1258 and being radiated from the power supply lead which I note is floating from the RRC case gnd. This probably means that a screened 12V lead will still radiate.

I would really like to eliminate this noise as it does degrade the station performance.

So; has anybody else observed noise coming from the RRC1258 12V power leads? Have you cured it?

Have the design team got any suggestions on additional filtering?  I can see what looks like a series TDK bifilar SMD choke and parallel capacitor on the 12V line as it enters the RRC1258.
Further I wonder if I am going to have the same problems with webswitches and the RC-1216H that I have?

73
John G4SWX

[g4swx]

I have been doing further measurements with a spectrum analyser and have attached two screen shots. They have been taken with an control RRC1258 (the lid on) with a Kenwood RC2000, speaker and microphone attached. The 12V power feed was threaded 3 times through a BN43-3312 balun core as close to the plug onto the RRC1258 as possible. The spectrum analyser pick up is simply a 5cm loop of wire fed by coax and held close to the RRC1258 box. Both attached screen shots are centred at 150MHz, one a 100MHz wide scan and the other 10MHz. As can be seen on the 10MHz wide scan the noise has significant peaks and troughs that can be 'tuned' by changing the applied voltage by 100mV or so. By tuning the 'noise' I can now show that it is easily detectable with an FT817 with a short helical antenna at 3m distance.
It seems to me that some of the internal filtering on the internal switching regulator is ineffective at VHF. Indeed there is a stronger peak of noise around 290MHz.

I am hoping that either somebody else has had a similar problem and resolved it or there is a simple component change from the design team.

73
John G4SWX

[sm2o]

Hi the emmisions is far below the values stated in the CE and FCC regulations. If you need get rid of the emmisions totally you probably need to replace the Switched regulator with a linear one. My experince is that the Ethernet cables is a bigger problem but you may already have changed to WiFi.

73 de mike

[g4swx]

Mike,

Thanks for the reply. I have already got rid of the Ethernet noise and for my application, 144MHz EME, the residual noise from the RRC1258 is still detectable and therefore I need to reduce/remove it.

It seems very clear from my measurements that the noise is very low at HF but not around 145/290MHz, it is quite possible that a simple modification could reduce it to an almost undetectable level.

Are there circuits available so that I can look to modify or replace the on-board switching regulator with an external linear one?

73
John G4SWX

[pd0swl]

Hi John,Mike,

I noticed exactly the same, as well as some "warbling" products from roughly 1.5 to 4.0 MHz.
Not too severe for me as my local qth has s9+ niose on any frequency, so the RRCs are still heroic devices for me! Without them I could as well stop this hobby.

However I took a look inside, it seems a big .470 smd choke is emitting the most noise and this seems to be re-radiated by the enclosure itself. I assume the choke is behind the switched regulator.
I'm in the process of faraday-caging my 2 RRC servers,see what that brings, but re-radiating enclosures is a tough one.

I would also be very intersted if the switched regulator could be exchanged by a linear one.
(If there is only a single regulator causing the noise).

Wifi has made my RRC link almost "transparant" but it would be very interesting taking the last drops of noise away.

(Besides these nit-picks from a die-hard user, the RRCs keep amazing me!)

73!
Paul
PD0PSB