C-QUAM test transmitter

by Oldeurope

 

In conventional externally excited AM transmitters, an unmodulated carrier comes from the control transmitter. In the transmitter end stage, the carrier is provided with the audio sum signal, i. left plus right stereo channel, monophonic amplitude modulated.

The C-QUAM transmitter differs from conventional AM transmitters in that the control transmitter delivers a phase-modulated C-QU carrier instead of the unmodulated carrier.
C-QUAM can therefore be used for all externally excited AM transmitters.

The transmitter output stage, in which the amplitude modulation takes place and the control transmitter can still operate with good efficiency also non-linear (for example, in operating class C).

C-QU is a phase-modulated carrier signal. If AM is modulated with suppressed quadrature (90-degree) carrier, quadrature modulation QU is obtained.
The difference signal, left minus right stereo channel, is quadrature-modulated.
If you add the AM signal to this, you get QUAM.
If you limit QUAM, C-QU remains.
C-QU comes from the controller.
If C-QU is amplitude-modulated, then C-QUAM.

A finished C-QUAM signal would have to be linearly amplified like a finished AM signal. Therefore, C-QUAM control transmitters have a stereo LF input, a mono-LF output for the modulator and a C-QU-HF output for the transmitter output stage.
C-QU can also be generated directly with phase modulation. So I do this with the following described C-QUAM test transmitter:

Block diagram of the C-QUAM transmitter

Differenzsignal gewinnung - Difference signal generation (and NRSC filter)
Pilot generator, Oscillator, Phase modulator, Amplitude modulator, Ferrite loop antenna

A few links that serve the understanding:

The Tech Zone with additional links

http://www.amstzone.org/

C-QUAM

http://www.wa2ise.com/radios/amstrjb.html

This one definitely brings an Aha experience

http://electronbunker.ca/eb/AM_StereoXMTR.html

Therefore, by means of phase modulation, a two-sided band signal with carrier in quadrature can be generated. pdf, page 8 of 18

(Not Found)

RMorg Overview of the AM Stereo systems in general. pdf at the end of the text

http://www.radiomuseum.org/forum/stereo_auf_mittelwell...

AM preemphasis pdf file see pages 4 of 24 below, 5 of 24 and 10 of 24.

NRSC-R10.pdf

I do not agree with everything. For this purpose, the descriptions of the modules of the transmitter are more important.

C-QUAM Lageplan der einzelnen Baugruppen - C-QUAM Map of the individual assemblies
LMC6484 - CMOS Quad Rail-to-Rail Input and Output Operational Amplifier
Begrenzer verstärker 4046 - Limiter amplifier section in 4046 PLL (PLL not used)
HF Endstufe - HF output stage
Modulationsanzeige - modulation display
Pilotton Oszillator und Teiler 4060 - Pilot tone oscillator and divider 4060
Sinusformung Pilotton 25 Hz - Sine shaping Pilot tone 25 Hz

The AM modulator and the transmitter antenna with ferrite bar

The amplitude modulation is effected by the modulation of the operating voltage of the output stage. Based on the classical anode screen grid modulation as "emitter modulation", because the transmitter output stage works in collector circuit.

 

Via the I-modulator, an emitter follower, the operating voltage of 2V5DC is fed to the output stage. A 100nF capacitor and a 47μH choke keep the RF away from the modulator.

At 100% modulation, the operating voltage that the modulator of the output stage provides varies between 0V8 and 4V2. The modulation control LED on the front panel goes out just before the 0V8 is reached. So the test transmitter can easily be adjusted.

 

The small operating voltage counteracts the drive of the ferrite antenna in serial resonance. The raw shows the RF voltage profile at the antenna capacitor and the voltage profile at the output in the balanced state. In the simulation I have the operating voltage from the smallest to the highest value graduated in different colors. You can see how the antenna circuit is sucking the first harmonic (fundamental wave) out of the square wave signal.

The emission via a magnetic antenna (here a ferrite bar as a transmitting antenna) does not require a counterweight (earth).

As a result, there are no brum problems when they are received. In order to avoid irradiation into the transmitter, the ferrite rod is placed horizontally about one meter away from the transmitter. The resonant circuit of the transmitter antenna is located in a piece of HT tube.

The circuit uses the BC557C in the collector circuit. Since the control voltage is less than the Z voltage of the BE diode, the operating voltage of the collector circuit can be modulated.

AM-Preemphasis, Stereo-Matrix and Modulator

 

With the Preemphasis I tried to approach the NRSC curve. Despite the preemphasis, it is easily possible to place two transmitters next to each other at a distance of 9KHz without interfering with each other. I tried it.
This works because audio frequencies over 4.5KHz in the sound frequency spectrum are relatively quiet. They are hardly blended down in the useful signal. Only when the neighbor is much stronger, splatters become audible. There is therefore no need to limit the bandwidth of the audio in the transmitter to 4.5KHz.
There are rumors that a stereo AM signal requires a higher bandwidth than a monophonic signal and therefore can only be used there in the USA because of the 10KHz grid.
However, in fact, the quadrature amplitude modulated signal or a corresponding phase modulated signal is not more broadband than the amplitude modulated signal. And at the one kiloherz more at channel distance in the USA it can not hang also, that it supposedly only there works.
It is of course possible but absurd to broadcast a C-QUAM stereo transmitter with 4.5KHz NF bandwidth and resulting 9KHz RF bandwidth. Even the narrow-band stereo reception has nothing to prevent.

Workupload preemphasis, sum and difference signal matrix and I-modulator LTspice

25Hz pilot tone generator for stereo recognition
To produce the 25Hz pilot tone, I use a 400P ceramic resonator in combination with the CD74HC4060. This makes the pilot tone generator free of any adjustment.

At Pin3 is a square wave signal with 24.7Hz. Three low-pass filters form a sine wave and a transistor brings the level to about 4Vpp.

 

 

Workupload Pilot sound processing LTspice

The sinusoidal shape suffices for the purpose. Since it is coupled to the Q-modulator via a high-resistance (100K), a coupling capacitor is superfluous.

 

The CD74HC4060 with its ceramic resonator is shown at the bottom left. Under the IC are still parts in the version since I had little space. The transistor on the upper right belonged to an RC phase shifting generator, now it makes the sine shaping for the 25Hz pilot tone.
Note that the pilot tone is included in the sideband, which is in quadrature to the carrier, as an under listening frequency with 6% -10% modulation degree. The stereo display on the receiver is not switched on by a simple amplitude modulation.



Carrier preparation: quartz oscillator, differential signal Q modulator and limiter amplifier:

In order to achieve high frequency stability, I use a quartz. I have made this one.

The XTAL oscillator is built with the CD74HCT4060. At Pin 11, a trimming capacitor is installed for accurate tuning of the frequency. At pin 7, the quartz frequency, divided by 16 and rectangular, appears. A series circuit is used to couple a filter circuit, which draws the first harmonic oscillation from the square wave signal. A capacitive voltage divider provides the now sinusoidal carrier oscillation with the appropriate amplitude to an emitter follower, which provides it with low-impedance buffering for further processing.

C-QUAM Modulator

 

Do the phase modulation like Steudler described in section 4.7.2 page 15 of 18. Via R3, the buffered oscillator voltage arrives without phase shift and unmodulated to the collector of Q1, the Q modulator.
At the base the oscillator voltage is in quadrature. C4 of which Xc = 33K and the basic voltage divider of R1 in parallel R4 together 776 Ohm push the phase by almost 90 degrees and divide the voltage to the required value for linear control of the transistor in the emitter circuit. Since the slope is proportional to the emitter current, the oscillator voltage appears in quadrature and with the L-R difference signal plus pilot tone amplitude-modulated on the collector. Both these add up and give the QU carrier signal superimposed with a difference signal and pilot tone.

The high pass from C2 and R6 removes the low frequency and the limiter amplifier in the 4046 the AM residues. The C-QU carrier is connected to the output of the limiter amplifier. The sine shape of the HF is very important in this case, so that no pulse width modulation occurs after the limitation, from which an undesirable amplitude modulation results.
Tip: Via pin5 to 16, inhibit, the unused stages of the 4046 can be switched off except for its limiter. Pins 3, 8 (GND) and 9 are connected. Pin14 signal in and Pin2 out. The remaining pins remain unconnected.
Via R7, the L-R difference signal is coupled with C6 to the emitter of the Q modulator. C5 does not cut the frequency response, but blocks the HF. Its optimal value I have determined with LTSpice. For the low frequency, this is a basic circuit. This does not rotate the polarity of the phase modulation.
The pilot tone is galvanically coupled to the Q modulator via the resistor R13. The modulation degree of the pilot tone mPilot can be calculated: With 100% channel separation, the difference signal level is 4Vss, as is the pilot level.
mPilot = R7 / R9 = 8K2 / 100K = 8.2%

 

Workupload C-QU carrier processing LTspice


A few important points:

One of the most important inventions of MW-Broadcast Technology is the channel grid. It allows interference-free reception with a high audio bandwidth at the same time by the use of notch filters in the receiving devices. Quartz stable frequency processing is therefore a must. A frequency in the MW channel raster is e.g. 720KHz. In Europe, the channel spacing is 9KHz. Subtract or add always 9KHz from the 720KHz to find a suitable frequency for your transmitter. There are also PLL kits on the net.

When using FM radio as a source, please take care that the 19KHz pilot tone, MPX and RDS are carefully removed from the low frequency.


Last update August 24, 2016 14:14 CET

Posted before 23rd October 2015 by oldeurope

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Saved from http://c-quam.blogspot.gr/2015/10/c-quam-pruefsender.html Saturday, October 14, 2017