Index

 

principle

block diagram

PTT switch

audio ampl.

microph. ampl.

equalizer

level cntl

level indicator

audio -bus to end 'spaghetty' at the radio table

(published in Electron #2, 2000)

 

 

 

Introduction

 

It this a familiar situation? In a QSO your party suddenly doesn't react anymore. When he returns after a few calls he tells you he took the wrong microphone.....

Or in a different version: you have been a radio-ham for quite some time and hate to part with your first transceiver, that nostalgic radio-system, that robust military set. To keep everything operational all are equipped with a microphone, Morse-key, headphones / speaker because plugs are different for each set, as are impedance levels, internal voltages etc.

 

This picture may be painted in almost every radio-amateur shack, including mine. With yet another transceiver to install and thereby completely loosing track through all cables on the table and, more important, loosing view to all the beautiful dials and displays, I decided to take action.

The following discussion is about my ham-bus, that is operational for quite some time now and really ended the 'spaghetti' at my radio-table once and for all.

 

 

 

Operational principles

 

Basic operational principles may be explained in a few simple lines:

 

-          all audio-outputs are 'translated' into a current source, allowing signals to be added and and this way to listen to more radio's at the same time.

-          all audio-inputs are connected to the same, low impedance microphone channel that is still allowing local levels to be tuned separately.

-          all 'PTT' inputs are controlled by a single line, still allowing complete separation between transceivers.

 

In this control system one bus line (with four wires) is connected to a small control box at the back of each transceiver and all control boxes are identical and are connected in parallel. The connection between the control box and the transceiver is dedicated and is equipped with the required connectors for that transceiver. The connection cable is connecting to a line-level input/output connector, most transceivers are equipped with. The four-wire line bus-line connects to the central ham-bus cabinet to control all sets.

 

Since the ham-bus cabinet is central to all equipment, all audio-processing is concentrated in this position and is acting on all transceivers. All your nice idea's as to an equalizer, notch, (DSP) filtering, microphone signal processing/compression, metering and digital mode I/O (FSK, packet) may be designed into the central ham-bus  cabinet. The central cabinet may also connect to all your peripheral equipment for recording an play-back and TNC / computer for all digital communication modes. The central ham-bus cabinet is further connecting to only one microphone, one speaker and a convenient send / receive switch (maybe foot controlled) to control all transceivers.

  

In principle the number of connected transceivers is unlimited. In practice this is limited to some extend by the power of the central microphone amplifier and the PTT control line. In the concept as below over ten transceivers may easily share one bus-line.

 

 

Block diagram

 

In figure 1 the ham-bus block diagram may be found. The 'match-box' is at the back of each transceiver and is showing all internal components (6). It is clear this box may be very small indeed. In my boxes a 5-pin DIN-connector is handling all connections to each transceiver, so all matchboxes will be the same; only the connection cable (and transceiver plug(s)) is typical to the transceiver. Each box is equipped with a contract strip to 'splice-in' the 4-wire bus-cable.

 

 

Figure 1. Audio-bus block diagram

 

 

The Morse-key symbol at the transceiver is depicting the PTT switch. Since all transceivers are in parallel, it  sometimes is more convenient to not allow all transceivers to switch to the transmit mode at the same time (e.g. in cross-band operation). To this extend a tumbler is added to the match box.

 

The speaker symbol at the transceiver is depicting the audio-output of the transceiver (headphones, speaker, line-output and the like). Try using an output as close as possible to the detector to prevent phase distortion when in FSK-mode. This output preferable is also just behind the volume control of the transceiver to enable controlling the output level to the bus to balance with other transceivers. A dummy headphone plug at the transceiver may silence the internal speaker, if any.

 

The 47 kOhm resistor in the match-box is changing the audio-output into a current source. The amplifier at the ham-bus is very low impedance at the input so all transceiver outputs may be put in parallel without mutual interference.

 

The microphone symbol at the transceiver is depicting the audio input to the transceiver. I noticed these input to be very different as to impedance and sensitivity. To this extend the ham-bus will put a constant signal level onto the bus, to be monitored by a level meter at the ham-bus cabinet. The match-box is containing a potentiometer to (once) set the bus level to the required sensitivity of the specific transceiver (e.g. at microphone level or line level).

 

 

PTT switch

 

In figure 1 also the complete (simple) diagram of the PTT switch may be found (upper right). A general purpose PNP transistor is directly connected to the bus-line through a 100 Ohm resistor (1 Watt) to current-limit an accidental short circuit (my supply voltage is 12 V.). PTT bus-line connects to the switch at each match box (to exclude a switched-on transceiver) and to a voltage divider that is controlling a general purpose NPN transistor. I noticed PTT inputs to be very different between transceivers as far as input voltages and currents were concerned. Since all needed a contact to ground for switching, the match-box transistor takes care of all other requirements while separating the transceiver from the bus at the same time.

  

The input filter section at the PTT switch will also be applied extensively at all other inputs at the ham-bus and is keeping HF currents out of the cabinet. This filtering actions should be well taken care off since bus-lines are running along all transceivers thereby acting as a (not so) nice pick-up antenna. Filters should be connected as close as possible to the ham-bus inputs.

When still some HF is leaking in, a sleeve choke will do a good job (run cable a few times through a ferrite toroide of 4A11 of '43' material). 

 

The type and number of centralized PTT switches is unlimited. For manual control I have a three position tumbler at the ham-bus with a click and a spring-loaded action and an additional foot controlled switch. Also the PTT control for all digital modes is acting through this circuit.

 

 

Audio amplifier 

 

The audio amplifier may be found in figure 2. Inputs and outputs are again equipped with the now familiar filter circuit.

The audio-input begins with a capacitor since some transceivers are presenting also a DV-voltage at the line output. This is followed by the virtual ground input of the first amplifier, at a gain of one (47 kOhm at the match-box).

Directly at the output of this amplifier is a second amplifier at a gain of three to take care of all digital modes and also a direct output for (tape-) recording purposes. The outputs are filtered again against HF break-in.

All operational amplifiers are of a general porpose type, in my system: NE5512 (dual) or NE5514 (quad.

 

 

speaker

 

 

headphones

 

 

Figure 2. Audio output amplifier

 

The central filter unit may be any type you like. I advice to make the filter switch-able as in the diagram.

In my ham-bus variation a have a selection of several filter types at my disposal. One is an order twelve, controllable, active high and low pass section to allow a variable band-pass section at any position in the audio band. Also two active notch filters are present as well as a DSP processor for speech control, also discussed at this web-site.

Some of these (digital) filters are allowing switching noise to seep through, which is why a filter is connected around the potentiometer.

 

The output section is straight forward again and constructed as a single-chip, two channel audio-amplifier. Many more solutions are possible and currently I am using a abondoned audio amplifier with small speaker boxes, that is directly connected to the potentiometer. More audio-sources are hanging around in the shack like FM- broadcast tuner, CD-players, that may all be controlled through this channel. 

 

 

Microphone amplifier

 

As already presented in the block diagram, the microphone amplifier is consisting of four distinguishable parts. The pre-amplifier, an (equalizing) filter, an automatic level control (AGC, 'processor') and a summing amplifier / line-driver.

 

Pre-amplifier and equalizer

The communication microphone is a much discussed topic. Some microphones are supposed to enhance your voice or give it a 'natural' sound and many more 'qualifications' may be heard on the waves, with microphone prices to range from a few bob to hundreds of Euros and more. In my experience a simple, very cheap electret microphone may do a very good job as well as surplus conference microphones that are offered in abundance at ham-fest. It is a good idea to start with as 'neutral' a microphone as possible since sound-coloring may always be performed afterwards. The latter is the basic idea behind this microphone channel with equalizer.

 

Figure 3 is presenting the microphone channel with the low-noise NE5533 as the pre-amplifier, designed for application with a dynamic (conference) microphone (output a few mili-volt) at a gain of about 30 dB. A small supply network is providing a power for an electret microphone. The amplifier is bringing the microphone signal to a convenient level of 50 mV.

 

 

 

Figure 3. Microphone amplifier and equalizer

 

Behind the preamplifier, the audio spectrum is sub-divided into three frequency ranges, with the highest frequency band (1,5 - 3 kHz.) and lowest band (20 - 500 Hz.) at a variable level control. The mid-band section has a fixed gain. The resistor network at the output summing amplifier ensures each channel to arrive at a gain of one with controls at the center position. The switch allows to cut-out the equalizer.

 

With this equalizer, many microphone characteristics may be selected (or corrected!). In my set-up I made many QSO's especially at difficult conditions, and fiddled the equalizer controls for best intelligibility at the other side for my particular voice type. In the end a setting has been found that is giving my voice the punch-through effect that is bringing 'the message' even with the S-meter not moving to the signal at the receiving side. This setting has not been changed ever since.

 

The opamps are general-purpose type again, e.g. NE5532 (dual) or NE5534 (quad). All filter inductors are of the micro-choke type, not much bigger than 1/4 W. resistors. These small indictors will exhibit some series resistance, that is part of the design; changing these micro-chokes for 'better' inductors therefore will yield 'less' results. 

 

Automatic level control and summation / buffer amplifier

Figure 4 is showing the automatic microphone level control (in some transceiver advertisements referred to as the 'processor') and the summation amplifier / line driver.

 

 

Figure 4. Automatic level control and line driver

 

The automatic level control is operating in the following way:

 

-          With the FET switched-off (gate at maximum negative voltage with respect to the source), the input voltage is attenuated by 100 x, followed by an amplifier at a gain of 1x. Total input to output 'gain' therefore is 0,01x.

-          With the FET switched hard on (gate voltage equal to the source), the input 10 kOhm resistor is connected to the virtual ground pin of the opamp. The total input to output gain therefore is at 15 / (10 + 15) = 0,6 x.

 

Dynamic control range of the amplifier is 0,6 / 0,01 = 60 x (36 dB) which is convenient at keeping 'normal' audio speech within limits.

 

The FET will be controlled negative as soon as the voltage at the output of the lower amplifier is more negative than 0,6 Volt (diode step voltage). This opamp is set at a gain of 200 x which may be calculated back to the input of the AGC circuit: 0,6 / 200 V. = 3 mV. This translates back to the input to 3 / 0,6 = 5 mV.  Level control will be at the end-of-range with the FET completely switched off. With more input signal, output will follow linearly. This level is reached with the input level 60 x higher, at 60 x 5 mV. = 300 mV. (the control range).

 

It is advisable to apply a FET with a low gate voltage range. This gate range is controlling the compression range.

Time constants for this circuit have been selected at 0,1 second for attack and 1 second for decay to smoothly control the output level without the 'pumping' effects often heard at the radio-bands.

 

With the nominal level from the pre-amplifier at 50 mV., whispering hams at -20 dB. and 'shouters' at + 16 dB will remain within control of this automatic level. Within this control range, output will vary between 45 mV and 300 mV to still allow for some voice dynamics. A switch will provide nominal signals without level control.

 

The potentiometer is set to a convenient audio level at the bus. The output buffer is also the right place to insert the digital signals and the play-back information from the recorder.

 

 

Level indicator

 

In the block diagram it was discussed the audio level at the bus should be set to a pre-conditioned level for all transceivers to reference to the same standard signal. Therefore we need an indicator to check for this standard level and control the level-setting when out of range.

I first planned to apply a big level meter, but finally decided I needed no more than a quick indication to see the signal is off, too low, just right or too high. This is conveniently designed around a two colored LED, showing none, green, yellow and red (yellow is red plus green). This small indicator may easily find its position at any crowded front panel. The indicator circuit may be found in figure 5.

 

 

 

Line-level signals are detected at the upper right opamp, that is wired as an active detector (no diode set-up voltage).  The (DC) detector output voltage is connected to the three level-detecting opamps at the left-hand site, the upper two as a detector with some hysteresis. The other input of each detector is connected to a resistive ladder network, setting the trigger voltages.

With the input voltage at 0,7 V. the first detector will trigger, lighting the green part of the combi-LED.

At 1,2 V. input, the second detector will trigger, also lighting the red part of the combi-LED. With red plus green lighted, the LED will show yellow.

At 2,4 V. at the input, the last detector will trigger, 'un-triggering' the upper detector so the green LED will extinguish with only the red light remaining.

 

While communicating, you may watch the yellow light for the right, nominal voltage at the bus line, with an accuracy of a few dB.

 

Any type of opamp may be selected for this function, that is capable of delivering a few milliamps to drive the LED's. A convenient type is housing four opamps in a single package.

Supply voltage for the ham-bus is set at +12 V. and - 12 V., but may easily be adapted at your convenience, provided you recalculate the reference voltages for the level detector.  

 

 

 

Bob J. van Donselaar, on9cvd@veron.nl