( literature by VU2DEV )
JOTA—40 RX. is a simple single conversion super HET receiver. OM VU2KGN is the motivator for this project. The general topology is based on 1985 ARRL handbook receiver. A 5—section xta| ladder filter at 12.0 MHz. IF [intermediate frequency] is used to select the band width based on senior HAMS inputs. Xtal filter is a little wider than necessary (about 4.5 kilohertz). The receiver circuit doesnot use any IC’s [Integrated circuits] for the sake of simplicity.
The receiver consists of Three single sided PCB assemblies, Namely RF Amp,RF Main board & V.F.O
The VFO circuit is based on 1978 ARRL handbook Info. The VFO gives three outputs for receiver, transmmitter and a frequency counter,
To cover 40 mtr band ile. 7.0 MH2 to 741 MHz, the VF0 covers 5.0 MHz to 4.9 MH2. VF0 tuning is by ordinary zener diode’s. This idea is from OM Hans Summers G0UPL. TheVFO is buffered by a two stage amp with feedback. This circuit shown below is adapted from a Hungarian radio. The original circuit had three outputs, Two main outputs and one counter output. So it is ideally suited to our application.
As mentioned earlier the VFO is a two transistor circuit and runs with an unusual voltage regulator. As the radio works an battery, battery voltage can drop up to 11V [Eleven volts]. The regulator used requires at least 2.4v input output differential to maintain regulation and of course reverse voltage protection is a must,Hence we can get a maximum regulated voltage of 11V — 2.4V — O.7V =7.9V and hence we have used a regulator 78LO5 with center pin [common] lifted above ground by 2,6V to get 7.6V .This is just to ensure ready available components with no adjustments.
7.0 MHz signals received from Antenna are Input to this PCB Assy. The Antenna signals are passed through A 2—SECTION BAND PASS FILTER to reject unwanted signals coming from Antenna. The circuit idea is from on old SIEMENS Radio [West Germany]. The circuit gives good rejection to IF FREQ of 12.0 MHz and image Frequency of 17.0 MHz. The Band width is left intentionally high at 350 KHz hoping that very soon we would be able to operate 7.0MHz to 7.2 MHz whereas presently we are authorised to operate only 7.0MHz to 7.1 MHz. The filter is low loss design ond the Inductors (Coils) are would on ϕ10mm Teflon® former with 24 S.W.G enamelled copper wire.
The RF AMP is the general purpose Amplifier with multiple feed back as given in ARRL hond book, This is a highly field proved design and there is scope for improvement by suitable choice of device for low cost, General purpose Receiver BC 547 is effective but experienced SWLS / Radio Amateurs could replace with more exotic/expensive devices for better performance. e.g 2N3866
The receiver circuit is realized on a separate single sided board. As given in block diagram this is a single conversion, high IF [12.0MHz] and for simplicity no AGC ond no lC`s. The image frequency is at 17.0 MHz [12.0 + 5.0] whereas wanted frequency is 12.0 — 5.0 = 7.0 MHz. So only a single section filter is used ahead of DBM [Double balanced diode rnixer]. The tuned circuit is shown below. The
antenna coil is rnode with 10mm teflon former ond 24 SWG wire is used for coil winding for low loss & high Q.
Coming to DBM block it is realised using 2no of trifilar transformers and T—1O Ferite core and 4 no of 1N4148 switching diodes. The idea at circuit is from 1985 ARRL handbook. This being a receiver diodes are picked—up from the same reel [Same batch], No further matching is done. The circuit is given below.
The mixer output is amplified in a single stage IF—Preamp using BC547. This is a very simple omplifier with feedbock 0nd no adjustments are necessory. The transistor collector directly drives the 3—stage Xtal filter.
The output from Xtal filter is amplified and buffered and given to the product detector. In our receiver A diode D.B.M is used as a product detector. The IF POST AMP circuit is given below. As the output is an emitter follower; It is low impedance and happily connects to the product detector.
A single stage Xtal oscillator with an emitter follower is used as a B.F.O. Now for some JARGON(please read this) 40 Mtr band is full of LSB stations only[kindly ignore other than HAM stations who use USB/ISB]. In our frequency mixing IF = RF + L.O that means the signals at 12.0 MHz IF are also L.S.B. please remember we are using a ladder filter with Xtals marked12.0 MHz. Ladder filter is fortunately a good L.S.B. Filter BUT ……..,…. But the center frequency of ladder filter is slightly below the marked frequency by 2 KHz or more.So to receive LSB signals the B.F.O shall be a WEE—BiT below 12.0 MHz.[may be ll,9985 or thereabout] for a simple receive—only application B.F.O. frequency is not very critical. As the filter is kept intentionally WIDE. The circuit of BFO is given below.
The product detector is driven by B.F.O and IF POST AMPLIFIER. The output is filtered OFF high frequency components by the so called diplexer and a big RF Choke. The resulting low level audio is amplified in a low—input impedance audio preamp to match the D.B.M. This circuit is from 1985 ARRL handbook. This common base amp output is further amplified in PNP Amplifier stage driving the volume control.
A simple push—pull amplifier is used to drive a speaker. Transistors are used instead of IC’s to make the availability of components easy. The output is more than 200 mw r.m.s good enough to operate JOTA activities in the midst of bubbling children. The output of amp can be connected to either 15 ohms or 8 ohms spkr. 15 ohms spkr will reduce the drain on battery whereas 8 ohms spkr would give more audio output.
A few receivers have been built using these circuits. As mentioned earlier,The VFO is on a separate pcb. Both the pcb’s have built—in reverse polarity protection. Performance of receiver is quite satisfactory while using a dipole antenna cut to 40 M—Band for normal HAM shack volume setting the receiver draws around 150 mA current on12V battery.Some HAMS have experimented using long wire antenna also. As this is meant to be a beginner’s receiver the pcb’s are slightly big to facilitate comfortable handling. There is no critical adjustments in the receiver. In fact antenna—coil trimmer was not mounted at all in a few receivers.
We would like to express our gratitude to OM NADARAJAN, VU2KGN for continuously encouraging us to take—up this project. |VU2DEV|
(photo of VU2DEV-courtesy -VU2JQX)