AM Demodulation Reducing Carrier Amplitude - Page 1
Forum section: electronics / rf-microwave

author: sparkytastic
author_rank: Newbie
timestamp: March 03, 2020, 06:10:30 pm
content: Trying to wrap my head around these concepts, I had a few questions.From my understanding:For AM communication, where a fixed carrier amplitude is used to encode a message signal, depending on the modulation index, there is a carrier amplitude present on the receiver. If the message signal is very low amplitude, it can be hard to gain the received signal given the "common mode carrier" that is present and often many orders of magnitude larger than the signal.Typical AM demodulation circuits have an antenna, preamp, and then go right into mixers to I and Q demodulation. Would it not make sense to use a difference amplifier and try to subtract or "null" out the carrier signal, to be able to have large amplification. Is this not done due to phase effects? Are there simple techniques that exist to automatically subtract out the carrier amplitude? Do these work well in practice?Thanks very much!

author: radiolistener
author_rank: Super Contributor
timestamp: March 03, 2020, 06:18:06 pm
content: the carrier is needed for demodulation. If carrier will be missing, you will need to use high stable local carrier (Beat Frequency Oscillator)

author: fcb
author_rank: Super Contributor
timestamp: March 03, 2020, 06:20:45 pm
content: In AM communication, the carrier amplitude is fixed frequency and the carrier amplitude is modulated with the signal (AM=amplitude modulation). I think you have confused AM and FM??

author: radiolistener
author_rank: Super Contributor
timestamp: March 03, 2020, 06:22:08 pm
content: AM with no carrier is known as DSB modulation.

author: sparkytastic
author_rank: Newbie
timestamp: March 03, 2020, 06:24:07 pm
content: Is it possible to convert AM with carrier to AM without carrier by subtracting or nulling out a fixed amplitude from the signal? Which would represent the received signal when the message is zero (flat)?

author: radiolistener
author_rank: Super Contributor
timestamp: March 03, 2020, 06:27:44 pm
content: yes, you can convert AM to DSB by remove carrier. But what is the reason for that? For DSB you will need to use high stable local BFO and even small BFO drift will leads to pitch change. Like it happens with SSB when you tune receiver.

author: sparkytastic
author_rank: Newbie
timestamp: March 03, 2020, 06:28:55 pm
content: Perhaps I didn't articulate correctly. Please let me know if I have a misunderstanding!We have a carrier:c(t)=A_c*cos(2πf_ct)And a signal:m(t)=A_m*cos(2πf_mt)Resulting signal is:s(t)=[A_c+A_m*cos(2πf_mt)]cos(2πf_ct)Is it possible to subtract out the A_c signal from the received signal so that the A_m signal can be gained?

author: sparkytastic
author_rank: Newbie
timestamp: March 03, 2020, 06:30:18 pm
content: Purely from an understanding point of view. If, for example, your ADC has lower bit depth, by applying gain you can see small signals. This is hard to do if carrier amplitude is so high. Can that be subtracted out and then gain applied?How would one remove carrier in terms of electronics? A diff amp?

author: radiolistener
author_rank: Super Contributor
timestamp: March 03, 2020, 07:56:13 pm
content: Quote from: sparkytastic on March 03, 2020, 06:30:18 pmit cannot help you, because sideband components have the same dynamic range. So, if carrier cannot fit into your ADC dynamic range, then sideband components also cannot fit.Quote from: sparkytastic on March 03, 2020, 06:30:18 pmYou can remove it with filter. If you want to remove it after frequency down shift to DC, the carrier is just DC component. So, you can just put some capacitor and it will remove DC component.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 03, 2020, 09:27:20 pm
content: OK this is old stuff (no digital here) and may not apply to what you are trying to do but here goes:I do not know if this helps but if you are trying to receive and amplify the SSB part of an AM signal you can use a SSB receiver.If you tune it correctly you can tune out the carrier and end up with only one SB.If it has good filtering characteristics you can use a SSB Rxvr to listen to AM. However most SSB radios do not have wide filters and you will not get good sound in commercial broadcasts.I suppose that if you were to use two receiving circuits, you could receive both SBs and effectively "null out"the carrier.Some military radios used this technique with AM reception just being really SSB. Can be a problem if you cannot tune out the carrier. A really big carrier may "desensitize"the receiver and even if you are tuned correctly, you may hear nothing much...On Transmit, they inserted a small "vestigal" carrier in the signal and ended up with a signal that can be received by AM radios. Really just a single SB with a very small carrier. This AM sounding signal is quite a bit more powerful(louder) watt for watt when received because the carrier does not carry much information.Wally

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 04, 2020, 01:29:21 am
content: Yes, that is the definition of DSB, the carrier is nulled so the A_c term is zeroed and the A_m cos(2 π f_m t) term remains.As mentioned, it can also be filtered, assuming f_m is greater than some minimum value (typically, meaning the signal has been high-passed; voice at 500-3000Hz is a practical case). It can be nulled at the receiver in the same way it can be added back in again, i.e. with a BFO, but the difference is the SSB/DSB BFO only needs to be frequency-accurate to work right; to actively null it, it needs to be phase-accurate as well. So, you either need a coherent source (rarely possible), or the BFO is PLL'd to the received carrier.Bonus of having a coherent BFO: you can do synchronous, baseband detection -- you can take 0 and 90° phase outputs from the BFO and demodulate the analytic signal. The resulting I and Q signals are then processed with DSP (since doing trig in analog isn't really feasible*).*Fine, Pythagorean theorem is almost practicable: square I, square Q, sum them, then square-root the result to recover the original signal (amplitude only).Tim

author: babysitter
author_rank: Frequent Contributor
timestamp: March 04, 2020, 09:50:34 pm
content: Let me add that you may have more or less stuff(tm) on your real world AM signal that makes the carrier and sideband amplitude change from the textbook kind:Fading, a natural effect. This is a time-variant change of path attenuation, due to the atmospheric path changing and components going different paths interfering with themselves. Fading might be frequency-selective, so the lower sideband might change different to the upper sideband. Doppler effects (due to reflecting areas moving) might push around your phase/frequency a tiny amount.Dynamic amplitude modulation: To conserve energy, some stations delay the modulation before the transmitter a bit, and control the carrier power so that during loud passages, the carrier level is high, during quiet passages, the carrier level is rather low, no one prevents a transmitter from completely switching off during silence.(Full disclosure: I was living next a AM station that used dynamic amplitude modulation very early. There was a public pool next to the station using its waste heat. My parents reported the switch to DAM caused a noticeable pool temperature drop.)73,Hendi

author: David Hess
author_rank: Super Contributor
timestamp: March 05, 2020, 05:38:03 pm
content: Quote from: sparkytastic on March 03, 2020, 06:10:30 pmAs far as I know, that is not the case; full recovery only requires the carrier to be of sufficient amplitude. I am not sure the dynamic range of the carrier to sideband would have a significant effect compared to other limitations on dynamic range; if the sidebands are that low, then received noise becomes significant no matter what the carrier level is. Note that AM receivers use automatic gain control while FM receivers use limiting.QuoteI would have said that typical AM demodulation is done with envelope detection.I and Q demodulation is how a Costas loop can be implemented which will produce synchronously demodulated AM and FM outputs from double sideband suppressed carrier signals in which case the amplitude of the carrier is irrelevant. Isn't that the whole reason to use a Costas loop?For narrower AM transmissions, sometimes single sideband with a reduced carrier is used. The reduce carrier allows phase locking of the local BFO to the carrier before demodulation. In some cases, apilot tone is used instead of a carrier.