R:190626/0617Z 31568@N3HYM.MD.USA.NOAM BPQ6.0.18
R:190626/0624Z @:GB7CIP.#32.GBR.EURO #:9878 [Caterham Surrey GBR] $:9878_GB7CIP

From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW

By G8MNY (Rewrite Aug 07)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

I have implemented a PEP Power meter in a large HF Valve PA, so that full carrier tuning is not required (used /P on generator). The circuit is loosely based on an old Radcom article (Jan 1986 page 46) that describes the principle of a conversion without much modification, & best of all maintaining the original calibration!


THE ORIGINAL CIRCUIT

┌─┤<├─────┬──100k─┬────┬───────┤<├──<+4V to 24V
_│_ D1 \│/ │ +│ 47u D4
┌─RV3─┐ │ === C4
│ 50k │ │ _│_
>─────┐ 1M 1M 3│\│8 IC LM358
ORIGINAL ├─100k─┴┬────)──┤+ \ 1 D2 5│\
RF SWR RV1 │ │ │ A >─┤>├─100k─┬────┤+ \ 7 D3
DETECTORS 200 C1│ ├──┤- / │ │ B >┬┤>├┬──┬──┬─> +VE SWITCHING │ === │ 2│/│4 C2 │ ┌─┤- / │ │ │ │ SWR
RV2 u1│ ├────)───┤├───┬────┤+ │6│/ │ │D5│ === METER
20K │ │ │ u1 10M === └───────┘ │ _│_ │u1
│ │ 100k │ │ C3│1u │ \_/ │C5
>─────┴─┬─────┴────)────┴────────┴────┴──────────────)──┴──┴─> -VE
_│_ └─────────────────────────────────┘


MY IMPLEMENTATION
As I did not have this specific 0V input IC, but plenty of normal dual FET ones (more RF immune than bipolar types), & I had an AC supply on the PA, I decided to use a simple straight forward ± dual power rail design...

20V AC >──────┬─────>>───1k2─────┬──┤>├──┬────┬────>+12V
_│_ PCB │ _│_ │
\_/ │ 12V /_\' ===100u
PTT │ │ │ │
RELAY +30V<──┼────R──┐ │ ├────┼────>0V
CIRCUIT │ ____│_____ │ _│_ │
100u === │Fail relay│ │ 12V /_\' ===100u
│ ~~~~│~~~~~ │ │ │
0V>──────┴───────┘ └──┤<├──┴────┴────>-12V

Mains Fail Relay
ORIGINAL /───────────────────────────────────────────────────\
METER >───' ─>>───┐PEP │\│+12V PEP┌───>>─ '──> +VE CONNECTION PCB ├───┬────┬──┤+ \ D1 │\ Rm PCB ORIGINAL FROM METER 100 │ === │ A >─┤>├─1k─┬────┤+ \ │ SWR METER SWITCH │ │ n47├──┤- / │ │ B >─┤ (211R in
RV1 === │ │/│-12V │ ┌─┤- / │ my case)
multi- │22n ├─────────4M7────┤ │ │/ │
turn │ │ X101 C1 === └────────┘
preset │ 47K u1 │
0V >>───┴───┴────┴────────────────┘
PCB

MAIN DIFFERENCES
The meter was used for 3 RF power functions as well as DC metering in this PA circuit, so the modification had to be faithful to all meter uses.

As there was HT metering, I found the modification was a SAFETY hazard when the PA is switch off, falsely indicating no HT! So I added a small double pole relay (fed from the +30V via a suitable R to get the right relay voltage) to revert the metering back to normal on no power. (useful for calibration by shorting the relay coil)

In a valve PA there is a lot of RF around, & finding a low RF field location for this circuit is important for correct operation. With the simple power supply on the same strip board as the op-amps, the number of RF carrying wires is kept to a minimum of just 4, reducing RFI problems. So only the input op-amp needed RF protection, with a 22n across the dummy meter resistor & a n47 across the op-amp input pins. I tested that this was enough by putting a series 10n in the meter input lead to bring in all the RF, & see if the meter moved at all, it did not. I also tested the circuit with an AF osc via a high value resistor to the input, to mimic peak SSB signals & checked it gave a flat peak reading response to > 2kHz. (the 22n & n47 affect this).

A major difference to the original published design, was to add gain of 101x
on the 1st op-amp & then attenuate by 101 with a series resistor (Rm=100x the meter value) to the meter. Without this, the op-amps are comparing tiny meter signal levels. So this system masks any small offset voltages, op-amp leakage, & RF pickup in the system. With ±12V supply there is plenty of amp headroom even with 101x gain. An accurate DMM is needed to test the 4M7, 47k, Meter, RV1+100 & Rm values. (see my "Odd Resistance Finder" bull for making up Rm)

HOW IT WORKS
The input load of the Multiturn Preset RV1 + 100R is set to be exactly the same resistance as the meter, in practice this can be finely tweeked later on for the accurate calibration.

Silicon signal diode D1 allows op-amp A only to charge up C1 @ 5mA max peak current, hence C1 is a small value to charge nearly instantly via the 1k, to the peak +ve voltage the original meter x 101, would see from the detector etc.

Op-amp B then accurately follows the C1 voltage & applies it to the meter via the 100x Rm resistor (1/101) maintaining the calibration.

Fine Calibration of RV1 is just to switch between normal & PEP (short relay supply) on a steady "hum less" carrier & adjust for the identical power reading. Check this at different power levels & bands.


See also my bul on "Meter Damping & Speed Up" & "QRO 1kW HF Metered Dummy Load"

Why Don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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