Using the Receiver

Antennas || VFO Mixer Injection Adjustment || Audio Gain Adjustment || Features Not Included
How Good is the Receiver?


Power Supply

A 1-amp, 12 to 13.5 volt, well filtered and regulated power supply should be used with the receiver. If the voltage drops below 12 volts, some of the IR switching may not work.

Total current draw of the receiver with no signal is 440 mA, but the audio amplifier (TDA2002) can draw instantaneous peaks of up to 700 mA.

The first indication of a weak power supply will be distortion in the audio amplifier. Wall warts (plug-in wall transformers/power supplies) are not filtered enough and will cause hum in the audio. A 1 amp wall wart will work, but be aware that most of them will cause hum in the audio and need additional filtering.

Several wall warts were tested with the receiver and we never found one that did not cause hum in the audio.

Antennas

The receiver is designed for use with Hamstick antennas on the inside of a condo/townhouse window. The RF amplifier and lots of mixer gain helps the receiver work in areas with poor reception.

A 17 meter ham stick will bring in all the bands of the receiver with equal gain. Hamsticks for the other bands, when used as a general purpose antenna, will show marked differences between the bands. For example, a 20 meter Hamstick will bring in 20 meters with a bang, but be poor on 40 meters. The VFO mixer injection level (see below) can be used to make any antenna work.

There is a 50 ohm pad at the output of the RF amplifier that can be changed to fit the receiver to your antenna farm. A 3dB 50 ohm pad comes stock with the receiver, but it can be increased to 6 dB, or even 12 dB, to reduce overloading the receiver when using very good outside antennas. See Circuit Details, RF Amplifier, "Adjusting Gain".

A third method of adjusting the receiver for outside antennas is changing the Gate 1 resistors at the first mixer. Gate 1 is marked at the MOSFETs in the first mixer. The stock value is 100K, and they can be lowered to 470 ohm for maximum dynamic range and stability of the mixer in high level signal situations.

Mixer Injection Potentiometer

The VFO injection level can be adjusted with the black PC mount potentiometer located at the first VFO amplifier on the VFO board. The pot is labeled "Gain Adjust" and is located directly above the FT50A-75 ferrite core, the RF choke for the VFO circuit.

The injection level can be seen visually with LEDs at two locations. The LED at the first VFO amplifier will vary in intensity as the pot is varied. A dim LED indicates lower VFO injection levels.

The second place the VFO injection level can be seen is at the first mixer. Both LEDs at the first mixer will visually show the injection level of the VFO signal.

When using a resonant outside antenna on 40 and 20 meters, it is often necessary to adjust the VFO injection level to where the LEDs at the first mixer are barely on. With nonresonant antennas, the LEDs at the first mixer are usually run at full brightness.

The VFO "Gain Adjust" pot can be lifted off the board and run to a panel mounted pot on the front panel of the receiver. The ungrounded lead of the PC mounted pot is run to the panel mounted potentiometer, connected to one outside terminal and the center one, with the other end of the panel pot grounded. Add another .01 bypass capacitor to the ungrounded side of the panel pot.

The gain of the mixer is changed with the VFO injection levels. Higher injection levels yield more gain. The gain and dynamic range of the mixer are inversely related. Higher dynamic range means less gain. More gain means less dynamic range.

The VFO mixer injection level adjustment is very handy to adjust the receiver to varying band conditions and antenna combinations.

Be sure to experiment with this pot when first using the receiver to become familiar with its advantages in receiver performance.

Check out the article in the May/June 2002 QEX article, "HF Receiver Dynamic Range: How Much Do We Need", by Peter E. Chadwick, on just how useful this mixer injection level in controlling the dynamic range of the receiver can be.

Audio Gain Adjustment Options

There is so much difference in hearing sensitivities between individuals that being able to set the audio gain to different levels was considered very important in the design of this receiver.

There are two places where the audio gain can be adjusted for individual preferences. The first place is the BFO amplifier injection level control.

There is a black PC mount potentiometer labeled "Gain Adjust" at the BFO amplifier directly above the MOSFET. This pot varies the injection level to the product detector. The injection level is indicated by the LED brightness at the BFO amplifier.

For minimum noise from the audio amplifier, this pot is usually run with the LED barely on. Increasing the injection level will raise the audio level a noticeable amount. For quiet ham shack locations, this pot will give all the variation you will need in audio levels.

The second place for audio gain adjustment is the "Gain Adj" capacitor at the audio pre-amplifier, located next to the SFU445A filter above the product detector. For maximum gain, a 2.2mfd electrolytic has been found to work best.

With the 2.2mfd electrolytic installed, audio output is adequate for noisy ham shacks, hamfests, and outdoor operations.

Values lower than 2.2mfd can be used to strike a compromise between an indoor level and the maximum available output.

Features Not Included

Some features on receivers that are not included are the following:

Noise Blanker

Receiver Incremental Tuning

Notch Filter/Q-Multiplier

Audio Filter

Just How Good is this Receiver?

Check out the article in the September/October 2002 QEX, "The DX Prowess of HF Receivers", and the design features of what he considers best for DX hunting. Most of those features are incorporated into this receiver.

Some of them are the following, taken from the article, page 38:

>Only single or double conversion is used instead of a chain of several mixers commonly used by other makers.

>A relatively low first IF that allows installation of narrow SSB/CW crystal filters with good shape factors to greatly attenuate out-of-band IF signals just at the front of the IF amplifier.

>The main IF selectivity of the crystal filters is very close to the receiver front end, which helps substantially to obtain high BDR and good IMD DR even for closely spaced strong signals.

>Ham-band-only preselector filters that substantially suppress strong signals outside of the ham bands and prevent receiver front-end overload and IMD.

>Narrow double-tuned preselector filters are switched by relays, so the receiver front end offers much better IMD response then when diode switching is used.

>A switchable HF pre-amplifier and switchable attenuator increase the range of receiver sensitivity adjustments, which allow the operator to adjust the receiver to particular propagation conditions and the receiving antenna in use. Note: This concept is implemented in the Electroluminescent receiver with the 50 ohm pad at the tail end of the RF amplifier, and the adjustable VFO mixer injection level.

>AGC is derived from the IF signal.

>A sharp IF crystal filter is close to the mixer and because of the relatively low IF, the crystal filter greatly attenuates out-of-IF signals. That helps to prevent receiver overloading by strong signals from outside the IF-filter pass-band.

Check out the article and compare!

You are welcome to send an image of your completed project. Any comments you have, be sure to email.

Enjoy and Have Fun - Don't forget your sunglasses!

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Last Update: 9/12/06
Web Author: David White, WN5Y