AM Transmitter and Receiver THD

radio8z's picture

Total Harmonic Distortion is a measure of non-linearity in an audio system and is calculated as the RMS voltage of all the harmonics divided by the test tone RMS voltage multiplied by 100%.. The THD of my AM transmitter over the air signal as received by two receivers was measured.

The test tone was an 800 Hz sine applied uncompressed to the previously describe "high efficiency" transmitter and the first receiver was a BC-1004-C with the IF bandwidth set to 10 kHz. The transmitter modulation and receiver AVC and audio volume were set to yield the minimum THD. The received audio signal THD measured 0.72%.

A second receiver, a Yaesu FRG-100, was similarly adjusted and the audio THD measured was 1.04%.

The results for the two receivers may not be comparable because the FRG-100 has a narrower bandwidth than the BC-1004-C which would tend to reduce the THD reading for the FRG-100 since some higher harmonics are attenuated.

The majority of the distortion most likely originates in the signal path from the transmitter antenna to the receiver antenna (added noise) and in the receiver electronics. A better AM receiver/detector would be needed in order to separate the transmitter THD from the rest (maybe another winter project in the works?). The data do indicate that the transmitter THD is less than 0.72%

Nonetheless, a THD around 1% for the total chain is not too bad.


Loud Music

I am guessing that the total harmonic distortion of much loud music recordings is around 97.8%.

I think it was found in the 1960s, that when a country singer's electric guitar was distorting due to a bad 12AX7 tube, the public liked the "grunge" sound and modern music was born.

The reason for having distortion free audio chains for recording and broadcasting is to deliver the artist's intended distortions with extreme accuracy.

Carl Blare


Not only is the THD going to be affected by receiver bandwidth and front end filtering and transmitter parameters, but your transmitting antenna system is going to be a huge role in measuring that THD.

You won't get the proper bandwidth given program material input if that antenna is a thin piece of wire compared to that TX feeding a nice fat 1/2 inch or larger conductor.


Antenna Affect on THD

In my case it does not despite having the resonant loading coil situation. I measured the audio response with the upper audio frequency response of the transmitter at the final modulator to be 21 kHz.and that of the transmitted signal previously to be up to 13 kHz. From this it appears that the antenna system BW is 26 kHz. The radiator is half length of 1/2 inch and tapers down to 3/8 inch.

If the limited bandwidth due to antenna Q were a factor this would make the THD appear to be lower. In today's measurement the receiver bandwidth used was 10 kHz which yields an audio BW or 5 kHz so this is dominant over the antenna BW. The combination of the 800 Hz tone and the 10 kHz receiver BW allows for the inclusion of 5 harmonics.

Carl, isn't it interesting that the fuzz and distortion intentionally added to music happened just as hi-fi and improved recording technology appeared. Our family's tape recorder in the late 50s was a mono "hi-fi" unit and I was disappointed in the distorted audio it produced. I replaced the heads, tubes, and even recapped it and adjusted the bias osc. and bought premium tape to no improvement. It was later when I made live recordings that I noticed they sounded very good. The distortion was not in the recorder rather it was in the electronic sources (radio and phono).

Lesson learned to "separate the variables" when possible.


Another Type of Distortion

Another kind of distortion that can be fun to analyze is "transient" distortion... the speed with which a mechanism or a system reacts to a sound event.

A fairly common observation of the "transient" nature of a device has been noted in the different sounds of condenser mics as compared with dynamic (magnetic) mics.

The smaller, thinner diaphragm of a condenser mic is able to move back and forth very rapidly, thus capturing a more instantaneous "attack" at the leading edge of a sound., giving a crisper more present sound.

The voice coil and heavier construction of a magnetic diaphragm is impeded by its own bulk and captures a leading edge of audio attack with a delayed reaction, giving a comparatively thicker or deeper emphasis to a sound.

The transient response of electronic circuits also measures how rapidly the circuit responds at all frequencies across the spectrum, known as the circuit's "flatness".

What kind of transient response is typical of an AM transmitter/antenna combination?

Carl Blare

Transient Analysis

There is a technique for measuring the dynamic and transient response of systems using what is known as the Dirac Delta function. This mathematically is a pulse of infinite height and zero duration with an area of one. If an approximate delta function is applied to a system the frequency response can be calculated by analyzing the output. A company I once consulted with made such a system to analyze bridges. It worked by "hammering" the bridge with a large electromagnet driven ball and recording the response. It literally was smacking it with a hammer and listening to it ring.

A pulse applied to a transmitter would essentially yield the frequency response of the system with high frequency response implying good transient performance. Other factors could be assessed such as overshoot and ringing.

Often an attempt is made to "critically damp" a system for fastest response without overshoot. For a microphone or speaker this involves presenting the device with a resistive load which accomplishes critical damping. Carl, have you ever heard of this in regard to microphones? An underdamped microphone will sound tinny and overdamped will sound muddy. It seems the best approach would be to have a small mass critically damped.


So Am I

I myself have been critically damped for several years.

No, I had not previously heard of that with regard to microphones.

Now that I think back upon various microphone sounds, I think I have heard under-damped and over-damped.

I have heard the word "damped" also used in regard to loudspeakers.

Carl Blare

Regarding Microphones...

I read a white paper regarding microphone response as it relates to proper impedance loading. As Neil stated, loading affects the damping of the signal produced by the microphone.

I use a Focusrite dual channel microphone preamp. Among other things, you can set the input impedance as needed to match the microphone application.

Taking the same mic and varying the loading on it, I can alter the characteristic sound.

This allows me to tweak the sound response for a given mic to match what I am recording. This has the effect of making one mic work like having several different sounding mics.

by MRAM 1500 

Charter Member - Association of Low Power Broadcasters

Chairman - ALPB

Log in or register to post comments