1W 27MHz RF power amplifier

The BLT50 transistor looks like it can handle a little bit more power, so I rise the bias current from 75mA to 300 mA, for 7.4V lithium battery as power supply, it dissipate 2.2W in idle. It's a little bit out of normal working range stated on datasheet. But, who care.

Worry for the high RF power would burn up the spectrum analyzer, 2 pieces of -3db attenuate is inserted between power amplifier output and spectrum analyzer.  

In initial design, the output is directly connected to the collector of transistor, in such configuration the max output power can comes to +27dbm (0.5W) with 150mA bias current. In later design the collector loading inductor is replaced by a 1:1.3 step up transformer and bias current rise to 300mA, which give a max power of +30dbm (1W). +24dbm is shown on spectrum analyzer because of the action of -6db attenuator. There are quite high level of higher harmonics because the transistor is driven beyond it's linear region. Some output filtering is needed.

The second -3db attenuator. Brought from a used equipment online store.

Input drive required to be +7dbm to give full output power of +30dbm.

A quick, simple, naive, 27MHz power amplifier experiment using BLT50 BJT

Brought a NXP BLT50 power transistor long time ago, today try to build a quick test circuit to see how it feels like. If this transistor works well, it would be put into the 27MHz remote control transmitter power amplifier stage. To maximize the voltage gain and reduce distortion,  the transistor is biased as class A at 75mA, and supply voltage 8.4V, it dissipate about 0.6W of heat, for a SOT223 package, it doesn't became too hot. At lower quiescent current, the maximum output power would be reduced.

The circuit is simple, no matching circuit, a small emitter resistor to provide a little bit thermal stability, the circuit is simulated before put into board, to reduce cut and try time.

I didn't want to spent a few days to wait for the PCB back from factory, so I use part of the board from 433MHz receiver project. The big inductor is the collector loading inductor, about 2uH inductance.

In 27MHz, 0dbm input, the amplifier produce an nice output spectrum. with -30dbc higher harmonics and 23dbm output power, which equal 200mW.

What is the range of this 200mW transmitter if it is put in the remote control transmitter?

May be I should go for a field test.

Long struggle of driving mini-circuit diode ring mixer

For a long time, driving the mini-circuit mixer LO port is a pain. Early attempts was using a transistor crystal oscillator to drive it, and later adding a 3:1 step-down transformer to drive it, and later adding series inductor to filter it. All attempts didn't give satisfied result. 

The problem is, when you probe the LO itself, the waveform looks clean, it's a sin wave, but once you connect the LO to the mixer, the waveform become bad, for bad, I means not looks like a sin wave anymore, it more looks like having many harmonics and the harmonics is just looks uncomfortably terrible. 

But why this issue stay so long? well, because the mixer seems work fine with those harmonics! It can works to produce the intended mixing product. Many version of remote control receiver was done.

I should try to fix this problem once again.

Waveform of mini-circuit ADE-1 mixer LO port, driven by 74LVC1GX04 crystal driver, at 27MHz. Overshoot is too heavy. Something needs to be done.

Spectrum view in IF port.

I also discovered that the LO harmonics leak through the IF port, the leaks like fireworks. All the way to 1.8GHz.

The breakthrough

 By using direct experimental method (blind guess and played around with it until it works), adding a big 470pF capacitor across the LO port to ground give a good result. The waveform looks more sinusoidal and less overshoot. Try adding shunt capacity is not the first time, but last time just not big enough! I guess the mixer LO port can be think of a small value inductor so adding a big cap to resonate out the inductance would do the job.

Shunting a 470pF capacitor on LO port smooth out the overshoot.

The LO harmonics leaks to IF port is significantly attenuated.

Last thing is to check the IF level, if adding the capacitor would make the IF level drop. And it doesn't drop. The conversion loss still keeps in -7dbm. 

Latest version of 27MHz RC Car remote control receiver

The latest version of 27MHz RC receiver, major change is replacing the expensive dsPIC30F MCU for a cheaper, newer PIC16F. This MCU included a DAC module can be use to control the variable gain IF amplifier, eliminated the external DAC used in previous version.

Pushing higher RF/LO frequency

Continue to push the active mixer test to higher frequency, without board modification, today's test start from 27MHz as it was tested yesterday, I gradually tune up the RF/LO frequency to 30MHz, 50MHz ....100MHz... 200MHz, the output doesn't drop in 50MHz, and when it come to 100MHz, the output dropped a little 1dbm compared with 27MHz, in 200MHz it still can make some conversion gain.

The active mixer still looks good in 100MHz, with a little bit harmonics, the left peak is the 1MHz -13dbm IF output.

A Not Bad test result from active mixer using BFR93 transistor

After get rid of the parasitic oscillation, test can be resume. A 27MHz -20dbm and a 28MHz -20dbm signal is feed to the RF and LO port of the mixer, resulting an output of 1MHz low side output signal, the amplitude is about -20dbm, which is 10dbm larger than using PN2222 transistor. The output is quite clean, not much spurious product. The two strong peak on the right side are the RF and LO signal, and IF output on the left side.

A clean looking downconversion using BFR93 transistor mixer.

Get ride of 1.2GHz parasitic oscillation

Don't exactly understand why, finally I could get rid of the 1.2GHz parasitic oscillation from the active mixer.

It is done by adding a 3.3pF capacitor across the collector loading resistor, as shown on screen, there are no high frequency spike above the mixer product signal frequency.

Output spectrum from low frequency to 1.8GHz

Parasitic oscillation using high Ft transistor

The active mixer does not work well above a few MHz of RF/LO, the output level can drops from -14dbm at 5MHz to -30dbm at 27MHz, so I replaced all transistor, these PN2222 general purpose transistor, to RF transistor BFR93A (Ft = 5GHz), surprisingly, even the inputs are all terminated in 50 ohm, there are a ~1.2GHz -20dbm parasitic oscillation at the output! If I could not get rid of this oscillation I won't be able to continue with any other test! I tried to added some smaller value supply decoupling capacitor but it doesn't help. The problem may come from the board layout. When I put finger on the board, it's not easy to disturb the oscillation, it just shift down the frequency a bit when finger tap on the upper side transistors. May be I should change to a lower Ft transistor.

Discrete RF circuit, is it useless?

In this modern time , most of the radio goes system-on-chip, i wonder what is the value of doing discrete RF circuit like using a transistor to build an LNA or mixer, is there any application of doing RF using discrete compoent ?

Gilbert cell active mixer fix

Discovered a mistake in the active mixer board, one of the inverting input was not AC grounded! It was left opened. That should be a major cause of the weird result of last test.  By shunting a 10nF capacitor across the inverting input to ground, the mixer works much better. As the screen showing a 1MHz tone mixed with a 1.1MHz tone, the 100KHz product signal is clearly seen and it's much stronger and spurious products is reduced, compare to the result from last test.

RF:1MHz -20dbm

LO:1.1MHz -20dbm

Troubleshooting the active mixer by circuit simulator

Trying to find out the cause of the poor performance of the active mixer, I switched from soldering desk to circuit simulator and it looks like a little good hints is found after some tweaking with the resistors values. Assuming all transistors are identical, if the base current of the lower side transistor and that of the upper side transistor are having a ratio of 2:1, the output would make a lower distortion and higher output amplitude. However, how to verify the base current of the transistors on the actual mixer board? I don't know. May be I should apply the bias resistor value from simulator to the actual board.

Gilbert cell active mixer test

Spent 2 hours to solder the Gilbert cell active mixer board, bias current is selected in 1.8mA at each of the output  loading resistors, and -20dbm for both RF and LO input, the result was disappointing. There are so many spurious signal product. The spurious signal product would be fewer for -30dbm input, but obvious the dynamic range is not enough.

-30dbm input power for RF and LO

-20dbm input power for RF and LO

RF/LO on the left side and output on the right side