THE OP-AMP:  The Op-Amp used for the signal meter circuit is a TL061.  An LF351 works just as well, however and has the same pin layout.  If you have a TL062 or similar the differing pin-outs are shown below so that you can design your circuit board successfully.

OP AMP Pin Layouts

Above, The Lowe HF-225 signal meter

You can download and save the above image to print out at a size that suits your particular meter movement

To make sense of 'S' units, the table below shows the relationship between 'S' units and terminated voltage - in microvolts:

Note how relatively small changes in voltage at the lower end of the scale ( in the S1 to S8 range) produce quite noticable swings in the readings, while really quite large changes in signal voltage at the higher end of the scale ( S9 to S+50 ) produce quite small variations in read-out.  This non-linear effect is quite intentional:  Increases in signal strength from S1 to S9 will produce dramatic improvements in the received signal to noise (S/N) radio while above around the S9+10dB signal level the receiver is approacing the best acheivable S/N ratio and further large increases in signal strength will make less if any improvement to the audible S/N ratio, so the S Meter does not really need to reflect these changes in such minute detail.

The signal meter is invaluable when making aerial adjustments and comaprisons as well as being useful for comparing the strength of various transmissions.


A Note About S-Meter Calibration

Although there is no set standard for S-Meter calibration, it seems widely accepted that the S9 reading is calibrated for a signal input of 50µV. (Although some manufacturers may calibrate S9 at 100µV). Additionally the gradation between each S unit is generally accepted as 6dB. i.e. S2 is 6dB greater than S1 and S9 is 12dB greater than S7 etc. As can be seen in the table above 6dB represents a doubling of the voltage received by the radio.

(It may also be worth pointing out that for a receiver to receive double the voltage from a transmitting station, the transmitting station would have to increase its effective radiated power level by four times.)

In real life the signal meter on many radios may not be accurately calibrated to 50µV at S9 and each individual S unit may not, indeed, accuratelt represent 6dB intervals. Some meters may represent the interval as 5dB or 4dB. However an S Meter's visual representation of relative signal strengths will still be very useful in judging differences between signal strengths of different stations and particularly useful with antenna experiments.

However, without an accurate signal generator, it will be down to individual judgement as to how this "home-brew" signal meter is calibrated!
 

Also see the  'DXing and Short Wave Radio' page for more about SIGNAL METER READINGS

DETAIL : CONNECTING THE SIGNAL METER TO THE LOWE HF150 - Or Other Radio

A minor modification to the HF-150 is required to enable an external signal meter to be connected.  The AGC (automatic gain control) line on the receiver's printed circuit board needs to be identified and an explanation was proveded by Lowe for the HF-150, but a similar connection could feasably be made to the AGC line of any other receiver.  This may be marked as a pin on an IC or next to a transistor or other component.  You may be able to find the circuit diagram for your radio in the handbook or be able to abtain one from the manufacturer or from a search of the internet.

A very thin wire can be soldered to the board at this AGC point and taken, via the 470k resistor, to a socket on the back or side panel of the radio.

I used a 2.5mm jack on the back of the HF-150 to facilitate easy connection and avoid the method described by Lowe below which involves cutting the PCB track to the earth terminal and exiting the signal meter feed by that terminal - thus the radio would no longer have an earth terminal.  Instead I carefully drilled a neat hole in the back plate and fitted the new dedicated socket for the signal meter.   The radio and the meter are then easily connected together by a short length of screened cable with a 2.5mm plug on each end.

This is the modification as described by Lowe:

	Modification for 'S' meter operation Modification of the HF 150 for use with the 'S' meter if your set has a serial number of 142617 or less it will require the addition of a 470k resistor. To modify your HF150 you will need to remove any power including batteries if fitted, lay the set upside down on a piece of cloth or similar to prevent scratching the case. Now remove the bottom panel;  With the front panel facing you locate the wire aerial connector, the black terminal being earth, if you now look at diagram No.1  you will see a thin track connecting this pad to the earth plane, you will need to cut this tiny track*, now you can solder a 470k resistor .25W from the black terminal to pin 16 of IC Q32 at the point where C97 is soldered onto the circuit board.  Please refer to diagram No.1. For sets after No. 142617 the 470k resistor will be fitted and all that will be needed is to cut the thin track to earth as to allow the agc signal out from the black terminal. * see my notes in the text
The panel below shows the modification required to be undertaken to the Lowe HF150 itself.  The original modification shows that the 470k resistor is taken from Pin 16 of Q32 to the Earth terminal.  I instead decided on the much neater option of fitting a dedicated 2.5mm jack socket on the back panel for the signal meter output:
* The Lowe explanation of how the signal meter is connected to the AGC line of the radio via a 470k resistor, and output through the earth terminal.  I instead opted to fit a dedicated 2.5mm socket to the back panel to take the feed out the the signal meter - the 470k resistor is therefore connected to pin 16 of I.C Q32 and taken directly to the 2.5mm jack socket.  This makes a much neater way of connecting up the external signal meter to the back of the Lowe HF150 I feel - as can be seen in the photo of the rear panel below:

Rear Panel - showing additional 2.5mm socket The photograph above shows the rear panel of the HF-150.  The additional 2.5mm signal meter output socket is mounted in a neat hole drilled just below the letter 'F' of the HF-150 logo top centre, above the battery housings. There is just one single thin wire to connect on to the circuit board within the radio at the AGC line.  This wire is taken to the 2.5mm jack socket via the 470k ohm resistor.  The body of the 2.5mm socket is grounded to the aluminium panel of the receiver.  It is then a simple matter to use a short piece of screened cable, terminated with a nice solid 2.5mm plug at each end, to connect the radio to the external signal meter - the centre conductor of the cable taking the AGC voltage to the meter while the braid screen simply connects the ground of both the radio and the signal meter casings together. Using this type of shielded connecting cable arrangement is probably not really necessary if you are running the signal meter and the radio from the same power supply transformer, as I do, and a single piece of hook-up wire between the radio's AGC and the signal meter circuit could do the job - but I used this method for neatness and reliability.  Using metal cased 2.5mm jack plugs looks more professional too!

ADJUSTING THE METER READOUT:  Once the meter is connected to the radio the sensitivity is adjusted by the 100k Ohm preset potentiometer - in essence this involves disconnecting the aerial from the receiver and ensure that no radio signal is being received (even hash or interference) and then to set the 100k preset so that the needle rests at minimum deflection i.e. as far left as possible or zero .

I hope you have a go at building this little project.  A signal meter can be a very useful tuning aid and I have found it invaluable for making various aerial adjustments and comparisons, as well as for general listening.



The LOWE SP-150

Hans Kröger writes regarding the Lowe SP-150 loudspeaker, audio notch filter and signal meter unit:  "Last summer I considered myself lucky to find a matching speaker/notch-filter/S-Meter at a HAM-fest, the SP-150. The missing instruction-manual I downloaded from the internet easy enough. When I connected the set to my HF-150 with an appropriate cable I found the S-Meter not to be working at all and the sound to be distorted at times. Some damage in the SP-150’s circuitry I thought and so began searching the internet for a source of repair.

That’s how I came upon the information that for receivers below a certain serial-number (just like mine) a “modification” has to be done by connecting the ground-terminal to an IC by means of a resistor (after having isolated this ground-clamp from the circuitry-ground first of all). That was easily done as well. While it struck me as absurd to rid the radio of all means to ground it, the obvious question has been on my mind ever since: How does the AGC-voltage that’s now available at the receiver’s ground-terminal (or at the tip of a 2.5 mm connector as per your approach, which I wish I’d read earlier), how does this AGC-voltage get to the SP-150’s S-Meter???

As per the manual a cable-connection must be made between the radio and the SP-150’s input. There is only one such connector at the set’s backside, which (sure enough) connects to Pin 2 of the IC described in the circuitry on your website (only in the SP-150 it’s IC 5 or 6). So where (and how) does one feed in the AGC-voltage? I cannot possibly be the only user facing this problem which I why I hope you’ve had reason to cope with this situation in the past and help me out.

What I will do in any case is re-connect the radio’s ground-lug to circuitry-ground, drill a small hole like you did and solder the 470K-resistor to a separate 2,5 mm-connector. Some advice and support would be highly appreciated! Many thanks in advance and all the best from Hamburg".

Solution - How to connect the audio and AGC signal meter drive to the Lowe SP-150: Well, fortunately the solution is relatively straightforward. Looking at the Lowe SP-150 circuit digram shows that both the audio from the Speaker Out socket on the rear ofthe HF-150 and the AGC voltage derived from the modification to the HF-150 described above is connected via a three conductor jack socket ('stereo jack') on the rear of the SP-150. The audio input socket serves this dual function which, sadly, is not made clear in the manual.

The three conductor ' stereo' jack on the SP-150 is wired as follows: The body of the plug is ground, the tip is audio and the centre ring is for the AGC voltage for the S-meter.