Demodulation of modulating square wave

[nauman]

Hi all

There is RF signal in frequency range of 240 MHz to 500 MHz which has been amplitude modulated by 155 Hz square wave signal. The problem is to recover 155 Hz signal while exact RF Carrier frequency is unknown (240 MHz to 500 MHz). Is there any ready made COTs solution available for such task?

Thanks & Regards

 

[berkeman]

Could you say more about the application? What is the received RF signal strength? Is there only one RF signal in that RF band, or can there be other signals that are not being AM modulated like that?

What is the Modulation Depth of this AM? Is this a real-life setup? If so, is the transmitter licensed? (Modulating with a square wave or OOK at 155kHz 155Hz is a bit strange...)

(Edit -- corrected my error)

 

[Borek]

at 155kHz

155 Hz from the original post

 

[berkeman]

Thanks Borek. That makes the sidebands less of an issue, but still a strange question, IMO.

 

[f95toli]

Modulating a 500 MHz carrier with a 155 Hz signal seems a bit odd.
That said, is there a reason for why you couldn't just use a power detector? If needed with a low pass filter afterwards.
See e,g,
https://www.minicircuits.com/WebStore/pd_coax.html

 

[tech99]

Just a diode envelope detector wil do it with low distortion.

 

[berkeman]

You guys with your inexpensive diode solutions. Sheesh. I was hoping to sell the OP a $50k spectrum analyzer + computerized solution. I already started to code it up...

 

[Averagesupernova]

Well, if you want to get technical, demodulating a modulating square wave implies that signal is to be demodulated prior to it modulating the actual carrier. So, how would one go about demodulating the baseband signal? ;)

 

[berkeman]

I'm guessing that it's some non-physical goofy OOK scenario. We'll see what the OP replies with.

 

[nauman]

Could you say more about the application? What is the received RF signal strength? Is there only one RF signal in that RF band, or can there be other signals that are not being AM modulated like that?

I do not have much details, however, i will try to explain what i know.
There is a mechanically rotating RF antenna which receives modulated or un-modulated signals in the 240 to 500 MHz range and provides an input to the associated receiver. When properly terminated, the field pattern of the antenna is essentially a cardioid.

By interchanging the feed and termination points of the antenna, a mirror image of the field pattern is obtained. In operation, the field pattern is switched at a 155 Hz rate thereby square–wave amplitude modulating the received RF. Bearing is obtained by detecting the modulated signal and using the resultant 155 Hz audio signal to drive a motor which rotates the antenna element until the field pattern crossover point is nulled on the transmitting station.

The problem in hand is to demodulate antenna output RF signal to get 155 Hz square wave audio signal as same is being used as a feedback control signal for motor. Also, exact frequency of received RF signal is also not known.

Thanks

 

[berkeman]

What is the typical signal strength at the RF receiver in dBm?

Are there any other RF signals in the area, or is this the only one in that RF band?

Did the diode detector suggestions above make sense?

 

[Averagesupernova]

Without any more information than we have, this sounds like time difference of arrival which generates an FM received signal. Easily demodulated with an inexpensive scanner.

 

[nauman]

Are there any other RF signals in the area, or is this the only one in that RF band?

There may be multiple RF sources with different directions in desired frequency band.

Did the diode detector suggestions above make sense?

what will happen to diode detector if there are multiple sources in desired frequency band with enough signal strength?

What is the typical signal strength at the RF receiver in dBm?

Presently, i do not know exactly but working to find out.

 

[Averagesupernova]

A little background story here would certainly help. No one does this sort of thing just because. There is an objective here, what is it?

 

[nauman]

A little background story here would certainly help. No one does this sort of thing just because. There is an objective here, what is it?

The antenna system is non operational due to non availability of motor feedback signal and to make antenna system working, 155 Hz modulating voltage is required.

 

[Baluncore]

It is difficult to work out what is really intended here, but by adjusting your description to comply with how it would be done in the DF world, I think the following interpretation is close to the intended system.

There is a mechanically rotating RF antenna which receives modulated or un-modulated signals in the 240 to 500 MHz range and provides an input to the associated receiver.

If you intend to DF a signal, you will need to know the allocated frequency, so you can tune your receiver to identify the required signal.

When properly terminated, the field pattern of the antenna is essentially a cardioid.
By interchanging the feed and termination points of the antenna, a mirror image of the field pattern is obtained.

My guess is that the DF antenna is a spaced pair of identical antenna elements, and it switches at 155 Hz between the sum and difference of the RF from those two element's. The output sum or difference not selected is terminated to Zo, so as to keep the hybrid coupler matched.

It is not necessarily the 155 Hz in the detected audio that you use for DF. The fast AGC voltage, or an RSSI voltage, will have the 155 Hz square wave with amplitude and polarity determined by direction.

That requires you have the master 155 Hz antenna switching signal as a reference, so you can synchronously detect the amplitude and phase polarity of the received signal strength.

In operation, the field pattern is switched at a 155 Hz rate thereby square–wave amplitude modulating the received RF.

Note that when you switch the phased receive antenna array at 155 Hz, every signal in the received 240 to 500 MHz band will be modulated by the 155 Hz signal. They may all have different directions.

Bearing is obtained by detecting the modulated signal and using the resultant 155 Hz audio signal to drive a motor which rotates the antenna element until the field pattern crossover point is nulled on the transmitting station.

Which is to say, the synchronous detector produces a DC signal that passes through zero volts as the signal crosses the boresight of the DF array.

Also, exact frequency of received RF signal is also not known.

Again. You will need to be able to tune to the signal of interest. A DF system cannot guess what you want to DF.

 

[berkeman]

The antenna system is non operational due to non availability of motor feedback signal and to make antenna system working, 155 Hz modulating voltage is required.

In addition to the great reply by @Baluncore (as usual), what is the mechanical rotational drive capability of your motorized antenna system when it is working? How quickly can it rotate to find the null in the antenna pattern (assuming that we can help you to decode the directional signal)? And at what frequency will you dither the aim of the antenna when it is on lock with the direction finding signal?

https://www.militaryaerospace.com/r...-initiative-on-wideband-phased-array-antennas

 

[tech99]

I do not have much details, however, i will try to explain what i know.
There is a mechanically rotating RF antenna which receives modulated or un-modulated signals in the 240 to 500 MHz range and provides an input to the associated receiver. When properly terminated, the field pattern of the antenna is essentially a cardioid.

By interchanging the feed and termination points of the antenna, a mirror image of the field pattern is obtained. In operation, the field pattern is switched at a 155 Hz rate thereby square–wave amplitude modulating the received RF. Bearing is obtained by detecting the modulated signal and using the resultant 155 Hz audio signal to drive a motor which rotates the antenna element until the field pattern crossover point is nulled on the transmitting station.

The problem in hand is to demodulate antenna output RF signal to get 155 Hz square wave audio signal as same is being used as a feedback control signal for motor. Also, exact frequency of received RF signal is also not known.

Thanks

What you are describing is similar to the Radio Compass on aircraft, apart from the operating frequency. So I am wondering if you mean an operating frequency of 240 - 500 kHz?

 

[Baluncore]

The problem is to recover 155 Hz signal while exact RF Carrier frequency is unknown (240 MHz to 500 MHz). Is there any ready made COTs solution available for such task?

Two signals from the antenna elements, or from a 180° hybrid, are passed through one receiver channel by switching between them at 155 Hz. If the two signals are different, a 155 Hz AM modulation will result. The polarity of the generated AM wave will indicate which input signal has the greater amplitude. If you have access to the original 155 Hz switch selection signal, then you could use a synchronous detector to extract the AM signal.

If you have NO access to the original 155 Hz switch selection signal, then you will need to regenerate it, which is more complicated.
Do you have access to that original 155 Hz switching signal ?
Do you have either an SDR, or a VHF/UHF communications receiver ?
Can you please identify the DF system by make and model number.

 

[nauman]

What you are describing is similar to the Radio Compass on aircraft, apart from the operating frequency. So I am wondering if you mean an operating frequency of 240 - 500 kHz?

Sorry, antenna operating frequency is still 240 - 500 MHz

 

[nauman]

Two signals from the antenna elements, or from a 180° hybrid, are passed through one receiver channel by switching between them at 155 Hz. If the two signals are different, a 155 Hz AM modulation will result. The polarity of the generated AM wave will indicate which input signal has the greater amplitude. If you have access to the original 155 Hz switch selection signal, then you could use a synchronous detector to extract the AM signal.

If you have NO access to the original 155 Hz switch selection signal, then you will need to regenerate it, which is more complicated.
Do you have access to that original 155 Hz switching signal ?
Do you have either an SDR, or a VHF/UHF communications receiver ?
Can you please identify the DF system by make and model number.

1. I do not have direct access to the system so i cannot say with surety but antenna working principle resembles with old AN/ARA–50 Direction Finder.
2. No, 155 Hz generator output is not accessible.
3. Presently, no SDR is available but will it not it be more complicated for this task? In SDR case, digital demodulated signal has to be again converted into analog signal for using as motor control feedback.

Are not there any commercially available AM demodulators with analog output which can do this task?

Thanks & Regards

 

[berkeman]

Are not there any commercially available AM demodulators with analog output which can do this task?

But seriously, tuned narrow-band ones, sure. Able to find a random signal over the broad frequency range you are specifying in the presence of other signals, not so much. That is probably why the SDR was suggested...

 

[tech99]

1. I do not have direct access to the system so i cannot say with surety but antenna working principle resembles with old AN/ARA–50 Direction Finder.
2. No, 155 Hz generator output is not accessible.
3. Presently, no SDR is available but will it not it be more complicated for this task? In SDR case, digital demodulated signal has to be again converted into analog signal for using as motor control feedback.

Are not there any commercially available AM demodulators with analog output which can do this task?

Thanks & Regards

I presume that the antennas point left and right of the vehicle, so that signals from ahead produce zero 155 Hz modulation. This is similar to an aircraft radio compass. If the antennas are switched at 155 Hz, then the output of a diode envelope detector will be 155 Hz and the amplitude will be dependent on the angle of the signal off centre line. In concept, had the 155 Hz been available, the signal direction could be displayed on a centre zero meter by reversing the meter at 155 Hz. A signal on the centre line then gives a centre zero indication.

Alternatively, instead of a meter, a DC motor could be used to rotate the antennas so that the incoming signal is equal on both loops. However, if the 155 Hz reference signal is not available, we can display an angle but there is no way of knowing whether a signal is left or right of the centre line. Maybe the switch for the motor could be locked to, or just driven by, the incoming 155 Hz, in which case the left/right will have to be manually set each time switching on.

Ambiguity will also arise whenever signal is lost. Left/right ambiguity can be resolved by turning the vehicle slightly to the right, and observing whether the indicated bearing decreases or increases. It should increase. With regard to receiver AGC, this should be useful and will not alter the bearing indication. And as others have mentioned, we presume that the receiver will be tuned to a particular transmission.

 

[Baluncore]

1. I do not have direct access to the system so i cannot say with surety but antenna working principle resembles with old AN/ARA–50 Direction Finder.
2. No, 155 Hz generator output is not accessible.

Am I correct in thinking you only have the AS-909/ARA-48 antenna unit ?

The Direction Finder Group (DFG) AN/ARN-50 shall consist of:
1. Antenna AS-909/ARA-48
2. Amplifier-Relay Assembly AM-3624/ARA-50, containing:
2a. Electronic Control Amplifier AM-1779/ARA-48
2b. Relay Assembly RE-339/ARA-48
2c. Interconnecting Box J-942/ARA-50
2d. Cover CW-455/ARA-50
3. Mounting MT-1955/ARA-50
4. Antenna AS-2755/ARA
Note: AN/ARA-50 installation consists of either item numbers 1, 2 and 3, or 2, 3 and 4.

ANTENNA AS909/ARA-48
FREQUENCY: VHF and UHF bands, 225-400 MHz.
TYPE: Cavity-backed slot.
DESCRIPTION: The antenna assembly consists of a directional receiving element, an antenna drive motor rate generator, a synchro generator, a lobing switch and the associated gear assembly for proper operation of these components. The antenna is a cavity-backed complementary slot radiator which is formed by the position of a rhombic-shaped metal plate located inside a conducting plane. The antenna element is terminated alternately at either end by the use of the antenna lobing switch thus reversing the radiation pattern in space ten times each second, and, effectively, squarewave modulating the incoming signal. The degree of modulation is proportional to the angle at which the incoming signal is received by the antenna element.

The antenna drive motor is designed to be driven by 115V 400Hz from a saturable core transformer, probably the output stage of a magnetic amplifier.

I believe the lobing control signal is from the amplifier unit to the antenna. It is specified to be a square-wave oscillator with a frequency between 75 and 200Hz.

 

[nauman]

Am I correct in thinking you only have the AS-909/ARA-48 antenna unit ?

DF system is connected with a radio receiver which is faulty and not working. Details of radio receiver are not known but this is the device which demodulates and provide 155 Hz audio signal to motor as feedback.

 

[Averagesupernova]

DF system is connected with a radio receiver which is faulty and not working. Details of radio receiver are not known but this is the device which demodulates and provide 155 Hz audio signal to motor as feedback.

Don't you think this is the place to start then? If you have the receiver in your possession, you likely know more than any of us.

 

[Baluncore]

Details of radio receiver are not known but this is the device which demodulates and provide 155 Hz audio signal to motor as feedback.

The AN/ANA-50 DFG switches at between 75 and 200 Hz. How do you know it uses only a 155 Hz lobing clock? I believe the master two phase lobing clock is generated by that unidentified receiver to control the antenna lobing switch. The returned audio is then synchronously detected by the unidentified receiver block, which controls the antenna 115V 400 Hz motor drive, which has the synchro to send the heading relative direction to the two cockpit display instruments.
The receiver block also contains a T/R switch for voice communication over the VHF/UHF channel using a separate TX antenna on the aircraft. The frequency control, RX mode, and press to talk mic is in the cockpit with the DF display, integrated into the pilot and 1st officer instruments.

To use the antenna alone;
1. You will need to generate the lobing clock to switch the antenna. Maybe 28 Vdc ?
2. You then need a receiver for the 225 to 400 MHz RF, that will produce the audio.
3. A synchronous detector, with an amplifier to drive the 115 V, 400 Hz antenna motor.
4. Some way of converting the antenna synchro signal into a direction display.
5. A vehicle like an aircraft that can support the antenna and provide power.
6. A test signal in the RF band that you need to DF, that might justify the exercise.

The antenna is just one part of a complex integrated group of modules in the avionics area and the cockpit. The last design specifications (including the power wiring connectors) for the AN/ANA-50 DFG were defined in MIL-D-38402A(USAF) 23 APR 87.