Riding the RF Gain control – part 1

Every so often one sees advice from experts on how to operate a communications receiver or transceiver for SSB reception on the HF bands.

Very often that advice is to adjust AF Gain to max, and adjust RF Gain for a comfortable listening level. This is argued today to deliver the best S/N ratio, partly due to delivering the lowest distortion due to IMD in the receiver front end.

The downside of this is that it has prevented normal AGC operation, so the operator must continuously adjust the RF Gain to compensate for fading etc, and the settings may be quite different for each station in a n-way QSO.

I cannot recall ever seeing quantitative support for the claimed improvement in  S/N or ‘quality’, so it seems that it is based on subjective assessment and there may not be quantitative evidence.

Now I was taught this method by my mentor 50+ years ago to operate a receiver he had loaned me… and it DID work in the specific case. It worth exploring why it did work, since this may be the root of the advice that is offered generally, whether appropriate or not.

Further articles will critically examine the advice applied to newer technologies.

Once upon a time…

There was a time when receivers had AGC systems that performed poorly, most commonly because they used an envelope detector with BFO injection for SSB and CW reception, and had AGC time constants quite unsuited to SSB Telephony.

In fact, my mentor’s instruction was for such a receiver, an AR8 receiver of WWII vintage which was essentially an AM receiver (including MCW) with BFO for A1 Morse code (CW) reception, that was its intended purpose. The AGC characteristic was tolerable for AM and MCW, and less suited to CW, but quite usable on mid range signals.

It could also receive SSB Telephony using the BFO, but BFO injection level was not adjustable and insufficient for strong signals so it was necessary to reduce RF Gain on strong signals to ensure reasonably good demodulation. The same was required for strong CW signals.

So, the instruction to set AF Gain to maximum and adjust RF Gain to comfortable listening level was a circumvention for the deficient means of demodulation of SSB Telephony, and poorly performing AGC system.

Next part

In the next part, we will explore a basic ‘conventional’ superheterodyne receiver with demodulator designed for SSB telephony.

Common mode choke for DSL line

Having decided to sack iiNet broadband because of recurrent under performance, I need to change VDSL2 modem as the one they supplied was locked to their SIP server (despite their assurances that there was no equipment lock-in).

I replaced it with a TP-Link Archer VR200v which seems to work ok except it is susceptible to disruption when I transmit on HF. The disruption is severe, it causes the VDSL2 modem to disconnect, and it takes around 5 minutes to reconnect.

Several different common mode chokes were tried, all of measured performance, and they all worked in that they eliminated the disconnection problem though they all resulted in small but acceptable uncorrected upstream errors. (Upstream errors are interesting since the upstream modem is 1000m distant.)

The ‘final’ design was chose as the core was just large enough to wind ordinary 4W modular cable through it. So the choke comprises a 2m length of 4W flat modular cable, one end wound 6 turns through a Fair-rite 2643102002 (FB43-1020) suppression sleeve, and RJ12 connectors crunched on in straight through pin wiring (ie reverse the plugs). I found the line jack in the modem would not accept RJ11 (4P4C) plugs readily (common with RJ45 sockets), it required an RJ12 plug. Continue reading Common mode choke for DSL line

Vacuum capacitors – construction implications for SRF

Vacuum capacitors are used for high end applications that require high voltage withstand and low loss.

Though they are called capacitors, and simple analyses treat them as a capacitance with some small equivalent series resistance (ESR), there is more to it.

Above is a view (courtesy of N4MQ) looking into one side of a vacuum capacitor. It consists of an outer cup, and a series of 5 inner cups progressively smaller in diameter. The other side of the capacitor has a similar structure but the cups site in the middle of the spaces between cups in the first side.
Continue reading Vacuum capacitors – construction implications for SRF

Vacuum capacitors – construction implications for Q

Vacuum capacitors are used for high end applications that require high voltage withstand and low loss.

Though they are called capacitors, and simple analyses treat them as a capacitance with some small equivalent series resistance (ESR), there is more to it.

Above is a view (courtesy of N4MQ) looking into one side of a vacuum capacitor. It consists of an outer cup, and a series of 5 inner cups progressively smaller in diameter. The other side of the capacitor has a similar structure but the cups site in the middle of the spaces between cups in the first side.
Continue reading Vacuum capacitors – construction implications for Q

Baofeng charger DC-DC converter

One of the insane things about some Baofeng hand held radios is that the charger cradle requires 10V DC, and malfunctions on car battery voltage.

The cigarette light adapter that came with a Baofeng GT-3TP was simply a pass through device and the charger did not work properly using the adapter plugged into a car.

The charger cradle electronics should include electronics to charge properly from a car, preferably from 10-16V. The implementation is a half-baked idea.

To solve the problem I have made an adapter cable using a small buck DC-DC converter off eBay (~$2), the converter is adjusted to 10V out.

I have used an Anderson Power Pole connector to suit my power distribution, you could use a cigarette lighter plug or some other plug to suit your needs. The output connector is a 2.5mm DC plug. Continue reading Baofeng charger DC-DC converter

A comparo of two bare light dimmer modules

Two bare dimmer modules sold on eBay with identical specification and similar price are compared.

Both claim to have zero hysteresis. Zero hints a lie!

Hysteresis is caused in simple phase control dimmer circuits at low settings because in each half cycle the trigger capacitor starts at a different voltage depending on whether the diac fired on the previous half cycle.

A serious issue with this snap-on effect is that if power is turned off at low power setting and re-applied, the controller may not switch on.

Above is type 1, a very triac basic phase control circuit. The red capacitor and resistor to its left are snubber components, the yellow capacitor, 4.7kΩ resistor to its left and the 500k pot are the phase delay circuit, the diac is just visible above the red capacitor. Continue reading A comparo of two bare light dimmer modules

Do we sound like a bunch of chooks or what?

Though ham radio enthusiasts often rail against CBers, with the eflux of time, it is more difficult to distinguish between the two.

From Innovantennas website, a caption to a pic of one of their antennas explains:

The 10-54MHz LDPA directional antenna at K4ANP. Sadly, Len caught one of the elements and vent it during installation but SWR 1:1.4 thorughout.

The emphasis of the SWR value is mine. They, as antenna ‘professionals’, do not seem to understand the concept of SWR, and that by definition it is a ratio of greater than unity, ie the big number goes first. In fact if they just rationalised the ratio and expressed it as SWR=1.4 it would be briefer, clearer and more importantly, actually correct.

Little wonder hams not really understand what they are buying acquire bad learning from those who might provide accurate information.