The "DigiSel" Preselector (IC-7700 & IC-7800)

by Adam Farson, VA7OJ/AB4OJ

The IC-7700 and IC-7800 are equipped with a "DigiSel" tracking preselector (one in the IC-7700, one per receiver in the IC-7800). The DigiSel Unit is a module which is placed between the RF BPF group and the RF preamplifier group. This location was selected to place the narrower filter (DigiSel) after the wider filter (RF BPF), in accordance with good RF engineering practice. The DigiSel covers the range 1.5 - 29.999999 MHz.

Note: The RF preamplifiers (Preamps 1 and 2) are locked out when the DigiSel is activated.

Fig. 1 is a simplified schematic of the IC-7700 DigiSel. The DigiSel is a one-pole tracking preselector employing a single high-Q tuned circuit, which is matched to 50Ω by capacitive dividers at the input and output. A single-ended Norton-type feedback amplifier (Q201) in the output signal path can be switched in or bypassed as required, to offset the insertion loss of the tuned circuit. This amplifier is optimised for linearity, so as not to degrade system IMD performance.  An attenuator is switched in at the amplifier input (via diodes D201/202) for gain trimming. The L and C values of the tuned circuits, and the divider C values, are selected by relay chains. The DigiSel tracks the receive frequency, but can be manually tuned via the front-panel [DIGI-SEL] knob.

Fig. 2 is a photo of the DigiSel Unit. The L components and relays can be clearly seen; the RF amplifier, with its feedback transformer, is on the top edge. The RF input and output connectors, and the bypass jumper, are on the right edge.

Fig.2: IC-7700 DigiSel Unit.

Fig. 3 is a partial block diagram. The received HF signal passes though the RX BPF group (BPF or LPF). The filtered signal is then fed to the DigiSel Unit, and either goes through or bypasses the DigiSel (Digital Preselector) filter. When the DigiSel function is activated, the RX signals are passed through the DigiSel input relay (RL21) and narrow bandwidth filter (D_SEL BPF) which filters out unwanted signals with its sharper frequency response. The filtered signals are amplified as required by the RF AMP (Q201) which offsets the insertion loss of the D_SEL BPF, then passed back to the RF Unit via the DigiSel output relay (RL2). The RX signal at the DigiSel output is then passed on to the RF Preamp on/bypass relay (RL1003).

Fig.3: Block diagram of DigiSel Unit.

Bandwidth and Insertion Loss/Gain Measurements: The DigiSel preselector will attenuate signals well removed from the frequency on which it is peaked. The bandwidth and insertion loss/gain were measured by applying  band-limited noise to the IC-7700 ANT1 input from an RF noise generator followed by a LPF with f_c = 30 MHz. The attenuation at f_o ± 250 kHz, and the insertion loss/gain at  f_o , were measured on the IC-7700 spectrum scope (f_o = receive frequency). Fig. 4 is a typical characteristic curve;  Figs. 5 and 6 are  typical screenshot, taken at 2  and 3.5 MHz respectively. Table 1 gives the results for the MF/HF range 1.5 ~ 28 MHz.

Fig.4.

 

Fig.5: Scope screenshot of DigiSel bandpass (160m).

Fig.6: Scope screenshot of DigiSel bandpass (80m).

Effect of DigiSel RF Amplifier on IMD₃ Dynamic Range (DR₃): As mentioned above, the RF amplifier has been optimised for linearity so as not to degrade the receiver's overall IMD performance when inserted. This was verified by measuring IMD₃ dynamic range (DR₃) at a frequency where the DigiSel exhibits insertion gain, indicating that the amplifier is in circuit.

DR₃ was measured in CW mode, at 500 Hz IF bandwidth, by applying two test signals at  21.000 and 21.010 MHz from a pair of RF signal generators . The generator outputs were fed via fixed attenuators to a hybrid combiner, and thence via a step attenuator to the IC-7700 ANT1 input. The IC-7700 was tuned to the lower (20.990 MHz) and upper (21.010 MHz) IMD₃ products in turn. The 2-signal power level was then adjusted for 10 dB (S+N)/N as measured on a baseband level meter connected to the speaker output, and DR₃ derived by subtracting MDS from the power of one of the two equal test signals for the "DigiSel off" and "DigiSel on" cases.

The measured DR₃ value for Δf = 10 kHz (averaged over both IMD₃ products) was 100 dB with DigiSel off, and 102.5 dB with DigiSel on. This suggests that there is no significant IMD₃ degradation due to the RF amplifier, and that a secondary effect may be occurring due to the reduction in reciprocal mixing noise with the DigiSel in the signal path.

Examples: The following scenarios illustrate situations in which the DigiSel reduces co-channel IMD₃ due to nearby HF broadcast stations.

• An 80m amateur signal f_o = 3600 kHz and two 75m broadcast signals  f₁ = 3200 kHz, f₂ = 3400 kHz. Both f₁ and f₂ will pass through the selected 3 - 4 MHz BPF.  One IMD₃ product falls at (2f₂-f₁) = (2*3400) - 3200 = 3600 kHz = f_o. The DigiSel attenuates the 3200 kHz signal by ≈ 35 dB and the 3400 kHz signal by ≈ 17 dB. If we average these attenuation values, the IMD₃ product is attenuated by ≈ (3 * 25) = 75 dB.

• A 40m amateur signal f_o = 7150 kHz and two 41m broadcast signals  f₁ = 7300 kHz, f₂ = 7450 kHz. Both f₁ and f₂ will pass through the selected 6 - 8 MHz BPF.  One IMD₃ product falls at (2f₁-f₂) = (2*7300) - 7450 = 7150 kHz = f_o. The DigiSel attenuates the 7300 kHz signal by ≈ 10 dB and the 7450 kHz signal by ≈ 15 dB. If we average these attenuation values, the IMD₃ product is attenuated by ≈ (3 * 12) = 36 dB.

Link: DigiSel Insertion Gain & Bandwidth.

References:

1. IC-7700 User Manual, pp. 1.10, 5-18.

2. IC-7700 Service Manual, pp. 3-1, 9-3, 10-16.

Copyright © 2010 A. Farson VA7OJ/AB4OJ (including images except where noted). All rights reserved. 

Last updated: 05/02/2010