Sudden Transceiver Low Pass Filters & TR Relay

The Low Pass Filters are an absolute must to insure that what is being transmitted does not contain unwanted mixing products, VFO or BFO harmonics or harmonics of the transmitted signal. I will provide data for both 40 Meters and 20 Meters.

Long ago I came across a seminal article from W3NQN which describes s series of Low Pass Filters designed specifically to address the second harmonic of the transmitted signal. I use these extensively and this is what is on the Sudden Transceiver. For your dining pleasure I have included the LT Spice Schematic and the frequency plot simulation so you can visualize how good they are. Note to do the simulation a termination resistor of 50 Ohms is shown. That resistor is not used in the actual hardware -- the 50 Ohms should be your antenna!

Later I will present a table of the winding data for the inductors as many hams would have a problem in going from inductance values to actual coils. But you should learn the how.

Above is the 40 Meter LPF Simulation and as you can see the cutoff is around 9 MHz so quite good. This same LPF would work with a 60 Meter rig as the second harmonic at 10 MHz would be nailed! Cleverly if you purchase a batch of 330PF and 100 PF Ceramic caps at 50 volts you can make up all of the cap values. A 100 PF in parallel with a 330 PF = 430 PF. Two 330 PF in parallel = 660 PF. Thus a total of six 330 PF and three 100 PF will give you what you need.

Now on to the 20 Meter Low Pass Filter. In this case I ran the plot out past the 2nd Harmonic (28 to 30 MHz). The LT Spice simulation is telling and shows a dramatic notch of the 2nd Harmonic.

 

For this filter you will need two 220PF Ceramic Caps, two 330PF Ceramic Cpas and one 47PF cap.

Core Winding Data

Inductance Al Value & Core Size Turns
1.34 uH 40 ~ T-37-2 Core (RED) 18 #26 enamel
1.22 uH 40 ~ T-37-2 Core (RED) 17 #26 enamel
0.670 uH 30 ~ T-37-6 Core (YELLOW) 15 #26 Enamel
0.608 uH 30 ~ T-37-2 Core (YELLOW) 14 #26 Enamel

(Note for those who do rigorous calcualtions the turns were rounded to the nearest integer. If you don't like my values --wind your own cores! )

 

T/R Relay

In my build I permanently connected the LPF to the IRF510 so that the antenna in receive is connected to the input of the Rx RF Amplifier. [This is the NC contact.] On transmit the antenna is now connected to the LPF. [This is the NO contact.] The T/R relay is activated by the PTT switch.