The values of C10, C11 and C16 were dictated by the choice of coil L2.
I used a 2 mH hf choke from my junkbox, as shown in one of the
photographs. C10 and C11 are the usual capacitors found in a Colpitts
oscillator. C12 isolates L2 from the DC on pin 6 of the mixer.
I found the BB104 type dual varicaps in my junk box. Two in parallel
were necessary to obtain the required tuning range. The lower
frequency was set by selecting C16. The upper limit was found
to be a bit high and this was corrected by adding R7. RV1 is the
tuning control. (The unmarked resistor between the wiper of RV1
and the varicaps is 120 kohm.)
of a direct conversion receiver is determined by a low pass filter
in the audio path. A high degree of selectivity is required here
because of the extremely strong station DCF39 at 138.82 kHz, only
1020 Hz above the upper limit of the band. I use a RC-filter with
three sections, each having a time constant RC=1 mS. At first
resistors and capacitors of the same value were used in the three
sections and this is the situation seen in one of the photographs.
Later I realised that a better response is obtained
when the loading of a section on its preceding one is decreased
and this resulted in the values seen in the circuit diagram. The
lower limit of the frequency response is set by the time constant
RC=(R17 + RV2)*C21 respectively RC=R16*C20.
The response of the metering circuit shows a maximum at about
36 Hz and is 3 dB down at 16 and 88 Hz.
The output of the low pass filter is fed to the two
sections of a dual opamp type UA747. The upper opamp feeds the
headphones. Volume is controlled by RV3 in the feedback path.
The lower opamp feeds a digital multimeter
that must be capable of measuring AC in the millivolt range up
to about 2 V. Preset resistor RV2 is adjusted when calibrating
the instrument. I choose to make the audio output, as indicated
by the DVM, 1000 mV when the instrument is placed in a field of
5 mV/m. The reading is linear up to about
10 mV/m maximum (2 V on the meter).