RECEIVER BUILDING BLOCKS
Broadband amplifier using a bipolar transistor.
This amplifier has about 20dB of gain, bandwidth of 100MHz and noise figure of about 7dB. The output transformer is 8 turns bifilar wound on a ferrite core. Use a medium power UHF transistor 2N5109, 2N3866, 2N4427 or similar. The transistor will need a small clip on heatsink. This amplifier is suitable for use in the front end of a HF receiver or as a post mixer amplifier following a diode mixer (see below.)
Mosfet R.F. amplifier.
Dual gate mosfet amplifiers offer high gain, low noise and good strong signal performance. This amplifier is used in the 2 metre to 10 metre converter, elsewhere on this site. BF981, BF961, 3SK88, 40673 or similar dual gate mosfets are suitable for this amp. Gain is about 25 dB, noise figure is 2dB or even lower.
Grounded gate FET amplifier.
The grounded gate FET amplifer has 10 to 15dB of gain, a low noise figure and good strong signal performance. This type of amplifier is widely used in VHF and UHF receivers.
MMIC (Monolithic microwave integrated circuit) amplifiers are easy to design and build. The MAR-1 has 18.5 dB gain at 100 MHz, 15.5 dB at 1000 MHz. See www.minicircuits.com for information.
BIPOLAR TRANSISTOR MIXER
The bipolar transistor mixer is widely used in broadcast receivers. It offers high conversion gain (15dB or more,) low noise, and needs very little local oscillator drive. This type of mixer has very poor strong signal performance.
The JFET mixer has good strong signal performance. Conversion gain is 3 to 6dB. Local oscillator drive = 2V pp.
DUAL GATE MOSFET MIXER
The dual gate mosfet mixer is one of my favourites. This mixer is used in the 2M to 10M and 6M to 30M converters described elsewhere on this site. Conversion gain is about 10 to 15dB. Local oscillator drive = 5V pp.
The passive diode DBM (Double balanced mixer) has very good strong signal performance. This type of mixer has a conversion loss of about 6dB. The mixer contributes very little noise, the noise figure is almost the same as the conversion loss. Diode DBM's are bi-directional. If an R.F. signal is applied to the R.F. port, an I.F. signal appears at the I.F. port. If an I.F. signal is applied to the I.F. port, an R.F. signal appears at the R.F. port. The same mixer can be used for receiving and transmitting, with a minimum of RX/TX switching. For optimum performance all ports should be properly terminated. The I.F. port is usually terminated by a 6dB attenuator or a diplexer (see below.)
SPECIFICATIONS OF SOME COMMERCIALLY MADE MIXERS
|Part #||L.O. Power||1dB Comp.||3rd Order Intercept||Frequency|
|SBL-1||+7 dBm||-||-||1-500 MHz|
This diplexer will ensure that the DBM I.F. port is properly terminated at all frequencies. The 50 Ohm resistors can be made from two 100 Ohm resistors in parallel. The capacitive reactance of C (XC) should be 50 Ohms. The Inductive reactance of L (XL) should be 50 Ohms. See table for component values for several common I.F. frequencies.
|Freq. MHz||C pF||L mH|
TRANSISTOR I.F. AMP.
The bipolar transistor I.F. amplifier has about 25dB of gain. AGC is applied to the transistor base. For amateur SSB/CW reception, forward AGC works better than reverse AGC. For forward AGC, the transistor current is increased until the amplifier gain decreases.
DUAL GATE MOSFET I.F. AMP.
Dual gate mosfets make excellent I.F. amplifiers. The AGC voltage is applied to gate 2 of the fet.
I.C. I.F. AMPLIFIERS
This I.F. amp. from the 80M SSB transceiver uses the Mororola MC1350P amplifier IC. www.mot.com
Other I.F. amps:
FET PRODUCT DETECTOR
DIODE DBM PRODUCT DETECTOR
LM380N AUDIO AMP.
Audio amplifier IC's
see www.national.com for pdf data sheets.
EI9GQ HOME BREW RADIO PAGE