9 LTspice, Radio transmitter ccts..

LTspice, Radio transmitter circuits.

This section contains several radio ccts in transmit mode.
Firstly download and install LTspice from www.analog.com
Then click on the names below to open the circuits
In case of difficulty downloading here are the circuits as a ZIP file

CW, this is TBD

Amplitude modulation (AM).

This section has three different methods of producing AM, the most efficient of these is superimposing the audio signal onto the supply rail, the other methods are inefficient as they need to be followed by linear power amps.

AM by superimposing the audio signal onto the supply rail (recommended).

This is the most efficient way of producing AM, as you can use a class C, or class D power amp with AM applied to the voltage supply rail.
The below circuit does this with class C.
Class D power Amps have their own section at the top level page of this website.

AM_by_applying audio to Vcc using linear.asc

AM by modulating the gain of a class A power amp (not recommended)..

This is possible but inefficient, as the following power amp has to be linear.
this is the same variable gain ampilifier circuit as is suggested for AGC in the amplifiers section, but the simulation instructions have changed.

AM by variable resistance FET in a linear amplifier

AM by unbalancing a diode mixer (not recommended)..

This is possible but inefficient, as the following power amp has to be linear.

Frequency modulation (FM).

The The example circuit here uses a frequency modulated Colpits oscillator, the frequency modulation uses a varactor diode. This is then followed by a class D power amp.
Note that there is a seperate section on Class D power Amps at the top level page of this website.
The class D power amp in the following example could be replaced by a class C power Amp, it is unlikely that you would use a class A, class B, or class AB power amp though, as these would be less efficient meaning bigger heatsinks and bigger power devices.

Single sideband (SSB).

There are 4Mhz and 38.4Mhz circuits here. The 4Mhz simulations work as intended selecting just one sideband.
The 38.4Mhz circuits, have too wide a filter passband at 16khz, and hence cannot generate SSB.

4Mhz crystal_filter_frequency_response_using_crystal equivalent circuit.asc

4Mhz_SSB_source_using_crystal_filter.asc

38.4Mhz crystal_filter_frequency_response_using_crystal equivalent circuit.asc

38.4Mhz_SSB_source_using_crystal_filter.asc

After a few circuits you may be able to ignore the directions written on the cct diagrams, as LTspice is reasonably simple to operate.